Improvements in Compound Object-glasses for Telescopes.
Year: 1892

page_text: Date of Application, 8th Oct., 1892
Complete Specification Left, 7th July, 1893-Accepted, 16th Sept., 1893
PROVISIONAL SPECIFICATION.
Improvements in Compound Object-glasses for Telescopes.
I, HAROLD DENnis Taylor, of 20 Boothain Terrace, York Optical Manager
to the Firm of Messrs. T. Cooke & Sons, Engineers & Opticians &c., of
Buckingham Works, Bishophill, York do hereby declare the nature of this
invention to be as follows:-
In the first place it is a fact, well known to opticians & scientific men that
ordinary object-glasses for telescopes, consisting of two lenses of ordinary silicate
crown glass and silicate flint glass respectively, are very far from being achromatic
in the sense in which a reflecting telescope is said to be achromatic.
Such an ordinary achromatic object glass of two lenses, when best corrected for
visual purposes, does not refract all the coloured rays proceeding from a distant
source of light, such as a star, all to one and the same point or focus. For the
bright green rays which constitute the central & about the brightest part of the
spectrum of white light, are refracted to the shortest focus or the focus least
distant from the object-glass, while all other coloured rays are refracted to focal
points lying at considerably greater distances from the object glass, the violet rays
having the greatest focal distance of all the colours. Vision through an achromatic
refracting telescope is thus necessarily dependent on a compromise, the eye
selecting out of these various foci that focus which makes, all things being
considered, the most powerful visual impression on the retina.
As an instance, some actual carefully executed measurements of the different
focal distances, according to colour, of the largest existing refracting telescope in
the world, will best bear out niy meaning.
Secondary spectrum, or variation of the foci for different colours, for the Lick
telescope, Mount Hamilton, California, as measured by Mr. James Keeler.
(Publications of the Astronomical Society of the Pacific, No. 9, 1890, p. 162.)
The rays marked by Fraunhofer's lines B (in the red) and F in the greenish
blue, are found to be united together at one focus, which is taken as zero. The
minus sign before some of the figures indicates a shorter focal distance for the rays
to which the figures refer, and the (+) plus signs a longer focal distance for the
rays referred to.
Distance of foci from

page_text: owing to the two colours being complementary) then all the other colours,
having longer or shorter focal points, are seen only as a more or less agressive
halo of considerable size surrounding the object, but of comparatively small
luminosity owing to the diffusion necessarily caused, by their being so much out
of focus.
Moreover, the foci of the actinic or photographically active rays (the blue and
violet rays) of the spectrum are diffused over such a distance (3 inches or so in the
above case) that no distinct focus for such rays is possible, and therefore an
ordinary achromatic object glass, when best corrected for visual purposes, is almost
useless for photographic purposes.
As a matter of fact, when it is desired to take a photograph with the great Lick
telescope, a large extra correcting lens has to be placed in front of the great object
glass, which correcting lens is designed to condense the blue & violet rays into the
most compact possible focus, which condition is necessarily attained at the cost of
reducing the focal length of the telescope by many feet & thus causing the focus to
be formed in a part of the tube which is inconvenient for access.
And
The practical realization of an object glass which will refract rays of all colours
much more nearly to one focus & be equally available both for visual purposes & for
photography has long been the aim of opticians & many scientific men.
certain combinations, mostly consisting of three lenses of different sorts of glass,
have been designed & calculated &, in some cases, actually carried out, which far
more nearly fulfilled the conditions of perfect achromatism; but insuperable
practical difficulties, such as the imperfect durability of the glasses or other
materials employed, have rendered their practical adoption well nigh impossible.
Now the combination of glasses, by which I hope to form an object glass which
shall be (i) almost perfectly achromatic, (ii) equally well corrected for photo-
graphic purposes as for visual purposes, & (iii) capable of practical construction in
large sizes & (iv) also of ordinary durability, is as follows:-.
1st. A convex or positive lens made of that type of crown glass which is
designated as 0.374 or No. 47 in the supplement to the price list or catalogue for
July 1886, published by Herren Schott & Gen, optical glass manufacturers, of
Jena, Germany. This crown glass has the following optical properties partly
as ascertained by Messrs. Schott & published in the aforesaid catalogue.
A to C
•3507
D to F
*7026
|
E to F*
F to G
F to H₁*
•3247
•5675
1.1564
* All partial dispersions or proportions of partial dispersions from E to F & 45
from F to H₁, herein given & marked by an asterisk are calculated as being most
probable by means of the dispersion formula of Cauchy, well known to students of
optics, whereby, when the indices of refraction for three or four sufficiently widely.

page_text: 1
l
In order to show how very nearly achromatic this combination is, I will give the
following figures & formulæ for central pencils, which will be readily understood by
scientific opticians. Assuming to be equal to I will add together the partial
dispersions for each region of the spectrum of each of these two positive lenses of
glasses 0-374 & 0·543 respectively.
ρ
1 Proportional parts combined dis-

page_text: dispersive effects upon different parts of the spectrum exerted by different sorts
of glass is usually termed) is almost precisely identical, as between the two
positive lenses combined on the one hand & the negative lens on the other
hand. Thus, if the power of the negative lens is adjusted to exactly neutralize
the combined dispersions of the two positive lenses for the spectrum space
C to F & thus allow the rays C to F to focus to one and the same point on
the optic axis, then all the other partial dispersions will be almost exactly
neutralized & all the various colours refracted to almost the same focus.
However, the figures point to a small deviation of the D ray beyond the focus
for C & F & a rather larger deviation of the very feebly luminous H₁ ray in
the extreme violet. For instance, if the figures given by Messrs. Schott are
strictly accurate then in the case of such as object glass of 15 feet focal length,
the yellow D ray would focus at a point nearly three hundredths of an inch
beyond the focus for C to F and the violet H₁ ray about six hundredths
beyond focus. On a proportionate scale, the great Lick telescope, before alluded
to, if constructed of the above three glasses in the same proportions would then
have all its foci for the various colours mentioned in the table concentrated
within a length of about one eighth of an inch instead of three to four inches,
as at present.
As regards the achromatism of this combination, it is immaterial whether the
lens of crown 0-374 is placed in front and the lens of 0.543 behind or vice versa,
but in any case it is necessary, in order to gain the largest possible amount of
refractive effect or focal power, that the biconcave flint lens should be placed
in the middle between the other two, and the two positive lenses so curved as
almost to fit into the curves of the negative lens, at any rate one positive lens
can be made with its inner curve the same as that of the negative lens, so that
the two surfaces may be cemented together if required, thus securing the passage
of a larger amount of light.
In the case of larger sizes moreover, since the curves of the concave or
negative lens must necessarily be rather violent as I shall presently indicate, and
as a hollow lenticular shape is very ill adapted to bear either the strain of its
own weight or any other mechanical strain, without sagging or bending, it is,
therefore, additionally necessary that this lens should be so shaped or have the
radii of its two surfaces so proportioned, that a ray of light in its normal passage
through & near to the edges of this lens on its way to the principal focus shall
impinge upon the first surface at an angle of incidence about equal to its angle
of emergence when leaving the second surface. Under these conditions it can be
shown that any mechanical bending or sagging of this lens can have little or no
effect in deviating the ray from its correct focus.
I will now give an idea of the curves for an object glass having an approximate
principal focal length of about 72 inches, the aperture being four inches, the
positive lens of crown glass 0.374 being placed first to receive the parallel rays &
the radii of the surfaces being given in the order in which the light passes through
them.
-
The sign indicates a convex surface & the sign a concave surface. The
radii (r & s) for each lens are expressed in inches.

page_text: The middle or negative lens is here supposed to be equi-concave & the
2nd surface of 1st lens fits into it for cementing. These curves very closely fulfil
the ronditions of achromatism & also satisfy those approximate equations relating
to spherical aberration which are well known to students of optics & mathematicians
& which must be satisfied in order that the combination may be "aplanatic" or
free from spherical aberration, so that all rays parallel to the optic axis and
impinging on the objective at any points from centre to edge may be refracted to
one and the same point at the focus.
1 1 1
But be it marked that the proportions between the quantities & for the
P P
three component lenses must not necessarily be in the precise relation I have here
indicated by way of an example, for the actual proportioning of these quantities in
order to secure the most perfect achromatism possible must ultimately depend upon
the exact optical characteristics of the three component glasses, which will
unavoidably vary slightly in the ordinary course of manufacture.
Let it be observed again that the above curves have been calculated to render
the objective aplanatic or free from spherical aberration as regards the yellow or
D ray.
It by no means follows that the objective would be aplanatic also for red
or blue light. But it need scarcely be pointed out that this objective cannot be
said to be as truly achromatic for the edge rays as for the central rays unless it is
aplanatic for light of all colours. But the usual formulæ dealing with spherical
aberration, as known to mathematicians, are not accurate enough in their nature,
especially in case of deep curves such as I am dealing with, to enable me to calculate
the curves adapted to render this objective aplanatic for all colours, at any rate
without entering into very tedious complications.
The curves necessary for the best efficiency of my invention can only be reliably
determined by actual trial upon a tolerably large scale, an objective of six inches
aperture for instance. It may then be found necessary to modify the shapes of the
lenses somewhat, constitently with the proportions between -, & being such as
1 1 1
..
P
P p
to yield the most perfect achromatism. Or it may be found necessary to separate
two of the component lenses by a small interval in order to make the combination
aplanatic for all colours.
Again, the methods by which the optical characteristics of these three glasses are
ascertained by Messrs. Schott, or would be ascertained by any other careful observer
with the aid of even the best of spectrometers, does not admit of such accuracy as
would enable the relative focal distances for any ray A, C, D, F or G, in the case
of a focal length of about 15 feet for instance, to be calculated without a liability to
an error of three or four hundreths of an inch or so.
For these reasons, therefore, I cannot give a precise statement of the proportions
& curves of my combination, which shall be reliable enough for working to, or
otherwise particularily describe all that is essential in the manufacture of my
invention until I shall have had the opportunity of making or getting made an
objective of six inches aperture or there abouts.
Possibly the actual carrying out of my invention on a larger scale may lead to
the detection of a still perceptible trace of secondary spectrum, which defect may
prompt me either directly or through the medium of Messrs. T. Cooke & Sons to
induce Messrs. Schott Gen to try to make such slight modifications in one or
more of these glasses as would perhaps entirely eliminate such remaining small
imperfections in the achromatism.
Or Messrs. Schott may, either accidentally & in the ordinary course of
manufacture of these glasses or else intentionally produce slight variations in
one or more of them which may possibly be advantageous for the purposes of my
invention.
Possibly also, they may turn out new meltings of these glasses, to which they

page_text: may give new numbers or designations other than 0.374,0-658 & 0-543 respectively,
as continually happens in the cases of other types of glass of their manufacture.
But I consider any three sorts of glass as coming well within the scope & limits
of my invention which shall severally display the optical characteristics herein
specified in such a degree of approximation as would justify any authority competent
to judge by being well acquainted with the physical & optical characteristics of
glasses & their liability to variations, in asserting that they are but minor or
accidental variations, or else intentionally favourable variatious of any or all of the
three sorts of glass herein specified.
Dated this th
1003
H. DENNIS TAYLOR.
COMPLETE SPECIFICATION.
Improvements in Compound Object-glasses for Telescopes.
I, HAROLD DENNIS TAYLOR of 20 Bootham Terrace, York, Optical Manager
to the Firm of T. Cooke & Sons, Opticians and Engineers &c. Buckingham Works,
Bishophill, York, do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :-
In the first place it is well known to opticians & scientific men that the so
called achromatic object-glass, composed of the usual crown & flint glasses is not
by any means, truly achromatic; that is, it does not truly reflect rays of all colours
proceeding from a white star to one and the same focal point. The blue & violet
rays, above all others, are refracted to a diffused focus far beyond the visual focus,
and therefore are useless either for visual defining purposes or for photographic
purposes.
As an excellent & typical example I here give a table extracted from the
publications of the Astronomical Society of the Pacific, No. 9, 1890, page 162,
which was published by Prof. James Keeler of the Lick Observatory. California.
This table gives the result of his careful measurements of the colour aberrations,
measured longitudinally along the optic axis, of the great Lick telescope. The
two rays of the spectrum B & F, in the red & green blue respectively are refracted
to the same focus.
The above figures agree very closely with the result of calculation and are
typical of all ordinary object glasses. It will be noticed that the eye selects a
focus where two bright complementary colours, full red & bluish green, focus
together giving a visual impression of white. The greater part of all the other

page_text: colours are thus useless for definition & if the object viewed is a star, the effect is
that a diffused halo of wasted light is formed around its image. Again the photo-
graphic rays about G are refracted to an indefinite focus about 1'65 inches beyond
the visual focus & therefore a visually corrected object glass of ordinary form is
not adapted for photography.
A photographic correcting lens has to be placed in front of the great Lick
object-glass to consolidate the actinic rays to a more compact focus, whenever a
photograph has to be taken. This shortens the focal length by many feet.
It is the principal object of my invention to produce an objective which shall be
free from the above colour aberrations & be, as a matter of fact, as achromatic as
a reflecting telescope, and therefore directly available for photography. I attain
this very important condition by combining together three lenses of three different
sorts of glass respectively, two positive lenses & one negative lens.
The three glasses employed by me are as follows:-
1st. A baryta light flint, as specified in the catalogue & price list for July 1886
of Herren Schott & Gen, optical glass manufacturers, Jena, Germany in the
supplement to which this sort of glass is numbered No. 58, and designated as 0.543
and described as having the following optical properties.
Index of refrn. for D ray.
1-5637
Reciprocal of its dispersive
power for interval.
C to F50-6
Partial dispersions or differences of indices for the following spectral intervals.
10000
3354
*7085
3309
5830
11857
The dispersions from E to F and from F to H₁ for the three glasses herein specified
are not given by Messrs. Schott but are calculated from their data by means of
Cauchy's well known dispersion formula.
2nd. A boro-silicate flint glass nearly answering to the type of glass designated
as 0.164 in the above mentioned catalogue of Herren Schott & Gen, but its optical
properties are as follows. This glass is designated as 0.658, by which term I shall
refer to it in the remainder of this Specification.

page_text: 3rd. A light silicate crown glass corresponding to the type numbered No. 47
and designated 0-374 in the supplement to Messrs. Schott's above mentioned
catalogue, in which its optical properties are given mainly as follows:-
I
Reciprocal of its dispersive
P1 Ps
Under these conditions it is evident to those conversant with optics that the
partial dispersions of these two lenses combined for any given region of the
spectrum, is obtained by-simply adding together their respective dispersions for
that region of the spectrum.
I herewith give a table in which in column 2, I give the partial dispersions for
baryta light flint 0.543 for those different regions of the spectrum indicated in
column 1.
$
In column 3 I give the corresponding partial dispersions for the same region of
the spectrum for the silicate crown glass 0-374.
.In column 4 I give the sums of the partial dispersions, for each region of the
spectrum, obtained by simply adding together the partial dispersions for 0.543
& 0.374 respectively for the same region of the spectrum, as indicated in
column 1.
In column 5 I take the combined dispersions as shown in column 4 for the
spectral region C to F as the unity of comparison & give the relative proportions
of the other combined dispersions for the other spectral regions in fractional parts,
in the same manner precisely as I have done in the case of the three glasses
considered separately.
In column 6 I give the corresponding proportional dispersions, in the same
sense as in column 5, for the boro-silicate flint glass 0-658. 1 need scarcely
explain that the perfection of the achromatism of the triple combination is made
manifest by the remarkably close agreement between the proportional dispersions
B

page_text: Figures calculated from Messrs. Schott's data by means of Cauchy's formula.
The "rationality" of dispersion, as it is generally termed, is in remarkably close
agreement.
Thus if the spectrometric observations and consequent data supplied by
Messrs. Schott are strictly accurate, then the only deviation of any coloured ray
from true focus by any amount of consequence is in the case of the H, ray at the
extreme violet extremity of the spectrum. As an instance I could show that in
the case of a 12 inch triple object-glass of the construction herein specified and of
15 feet focal length the H, ray would focus at a point about 6 hundredths of an inch
beyond the focus for the brightest colours, supposing the object-glass is properly
corrected for achromatism.
In the case of an ordinary object glass of 15 feet focal length, the deviation of
the H, ray beyond focus, supposing that the rays C & F are refracted to the same
focus, would be about 85 inches or 14 times as much.
I need scarcely point out that the achromatic correction & freedom from
secondary spectrum of my objective is independent of the order in which the
three lenses are placed, provided they are in contact or nearly so, but depends upon
the proper ratios between the values of
1
&
P1 P2 Pz
The order in which they are placed and the forms of the lenses are subordinated
to other very important considerations as I will now explain, after first giving the
actual curves &c., which I have found to give the finest results & greatest practical
advantages which it is possible to secure for my invention.

page_text: The accompanying drawing shows, in Fig. 1, a full size cross section of an
objective of inch aperture and about 108 inches focal length for parallel rays,
which are supposed to enter the objective from left to right.
Then the three lenses are placed in the following order―
1st. The positive lens of baryta light flint glass 0.543.
This is L₁ in the drawing.
2nd. The negative lens of the borosilicate flint glass 0.658.
This is L, in the drawing.
3rd. The positive lens of light silicate crown glass 0.374.
This is L, in the drawing.
I will now give the radii of the curvatures, for a focal length of about 108 inches,
n the order in which the light passes through the objective, indicating a convex
surface by a plus (+) sign before its radius of curvature and a concave surface by
a minus (-) sign placed before its radius of curvature.
These radii are expressed in.inches.
These curves give the following very important advantages:-
1st. Only four pairs of tools are required, since the inner contiguous pairs of
surfaces are worked to the same radius.
2nd. The achromatism of the objective can be perfectly secured, or else varied
to suit different sorts of eyepieces, by simply altering the radius of curvature of
the back or concave surface of L, in the usual way, without any disturbance taking
place in the correction for spherical aberration. For the light passes through this
6th surface so nearly perpendicularly that its effect upon the spherical aberration is
almost nothing.
3rd. The objective is free from spherical aberration for all colours alike, a
condition of considerable importance as I pointed out in my Prov. Specification.
This condition depends upon the separation between L, & L, not being allowed to
vary more than 10 to 15 per cent. from the amount specified above.
4th. The objective yields the largest possible field of view, the oblique images of
stars being quite free from " coma or side flare.
"
5th. This construction admits of all six surfaces being tested as to their state of
figuring either by a direct or an indirect method as I shall shortly explain. This
condition is of immense importance in the practical construction of the objective.
In the case of a double objective there are two convex surfaces and in the case of
a double objective giving the largest possible field of view, there are three convex
surfaces, none of which, in either case, can be individually tested for errors in
figuring which are two small to be detected by an spherometer. Consequently a
serious amount of time is often lost in refiguring the wrong surfaces.
6th. The condition that the rays shall enter & leave the surfaces of the
mechanically weak flint lens at approximately equal angles is tolerably well
fulfilled. I pointed out the importance of this in the case of large objectives
liable to flexure in my Prov. Specification.

page_text: The actual radii for an objective of 6 inches aperture and 108 inches focal
length having been given, to ascertain the radii for any other focal length is simply
a matter of proportion.
If specially desired, the above relation of 1 to 18 between aperture & focal
length can be made even 1 to 15 if the central thicknesses of the two positive
lenses and the edge thickness of the negative lens are sufficiently increased; but
this greater proportion of aperture to focal length is not to be advised for any
objective of more than 5 or 6 inches aperture, for it increases the difficulties of
working and the absorbtion & loss of light due to thickness and more oblique
incidence at the margins of the surfaces.
The lenses should be ground & smoothed in the usual way & may then undergo
any of the rough polishing processes. After that they must be figured by careful
pitch polishing such as is generally adopted by opticians.
L₁
1st surface should be figured with the lens uppermost.
Li {2nd surface should be figured with the polisher uppermost.
L₂ {
L
3rd surface should be figured with the lens uppermost.
4th surface should be figured with the lens uppermost.
5th surface should be figured with the polisher uppermost.
6th surface is better figured with the lens uppermost.
The 3rd 4th & 6th surfaces, being concave, can be directly tested for figure by
placing an artificial star close to the centre of curvature & examining the reflected
image of the same with an eyepiece, which may be a low power in case of the
comparatively flat 6th surface, but in the case of the 3rd & 4th surfaces the
eyepiece should consist of a rather high power microscope objective of tolerably
large angular aperture, and corrected for spherical aberration. For an ordinary
high power eye-piece, either of the Huygenian or Rannden construction gives rise
to such an amount of spherical aberration in the case of a reflecting surface of such
large relative aperture, as to disguise the effects of minor irregularities of surface.
The ordinary microscope objectives sold screwed together in sticks or rolls, answer
the purpose very well.
Before trying the triple objective as a whole the lenses should be edged down in
a proper dioptric centering lathe & fitted into a temporary brass cell rather larger
than the cell which the objective is finally intended for.
Lenses 1 & 2 should always be just kept apart & prevented from scratching one
another by means of three pieces of the thinnest tinfoil carefully gummed or cemented
at equidistant intervals round the edge of either of the lenses just outside the
limits of aperture. All superfluous cement should be squeezed out from underneath
the tinfoil, which should be carefully smoothed down to the glass.
The brass ring shown in Fig. 3. in plan half size, and in Fig. 2 in cross section
should be placed between the 2nd and 3rd lenses.
It is of the utmost importance that the three projecting parts of this ring should

page_text: be shaped to the curve in the lathe & be kept of exactly equal thickness, measured
parallel to the optic axis, and equal to the amount of separation specified in the..
above schedule of curves.
Of course this thickness, which determines the separation between L, & L., is-
directly proportional to the focal length required, as in the case of the radii of
curvatures.
If the brass ring is too thick at first, it may be reduced at the projecting parts-
by judicious grinding between the two tools to which surfaces r, & r, have been
worked, followed by scraping, taking care to keep the thicknesses equal by careful
callipering. For if the thicknesses of the projections on this ring are unequal,
then a side flare or eccentric " coma "will be noticeable at the focus when trying
the objective on a star.
If the flare extends towards the right hand when observing the star image, then
the lenses L, & L, are relatively too far apart on the left hand, or, in other words,
the brass separating ring is relatively too thick on the left hand, where it must
therefore be reduced.
The three studs or projections on the brass ring have to so lie in the cell as to-
come in alignment with the three pieces of tinfoil separating L₁ from L.
Also the two opposing shoulders of the cell should be provided with three-
shallow & equidistant projecting studs to correspond with the two other sets of
contact points just described. So that when finally fitted up, as well as when,
trying the objective, it may be just gently confined free from shake and the lenses.
held up to one another at three equidistant points round their edges.
The counter ring keeping the objective home in its cell should be fastened or
screwed to the body of the cell at three points intermediate between its three-
projecting studs; this secures elasticity in the confinement of the objective.
Supposing the objective is in its temporary cell for trial upon a real or artificial
star & certain irregularities of figuring are evident at the focus, and provided it is.
known that the bad figuring does not reside in any of the concave surfaces, then the
convex surfaces may be thus tested.
For testing the 2nd surface, let the objective be taken apart again, and then let
a few drops of a liquid having a refractive index approximately equal to that of
glass-spirits of turpentine answers very well,-be poured into the middle of the
3rd surface, the first lens being then replaced as before. Then the thin interspace-
between L, & L, will be filled up with the liquid. It need scarcely be pointed out
that if the objective be now tried again, then all the faults of figuring before
visible at the focus will now more or less, totally disappear if they exist in the
2nd surface; for the index of refraction from the glass of L₁ into the liquid is so.
very nearly equal to 1, that the index of deviation is reduced to practically nothing
& therefore errors of figuring can exert no perceptible effect at the focus. If,
however, the errors still appear at the focus, it is evident that either 1st or 5th
surfaces must be at fault, or both.
To test the 5th surface take the objective apart, dry & clean the 2nd & 3rd
surfaces, place the brass ring between the 2nd & 3rd surfaces, & fasten three
equidistant pieces of thin tin foil onto the edges of the 4th or 5th surfaces to.
prevent scratching or cohesion, then put the refracting liquid between the 4th &
5th surfaces & try the objective again. It will then be found that although the
aberration and colour correction appears somewhat different, yet any irregularities
of figuring will have disappeared if they are present in the 5th surface, but if they
are still visible then the conclusion is that the 1st surface is at fault. Thus all the
surfaces may be tested.
The best refracting liquid to use is perhaps a mixture of 4 volumes of spirits of
turpentine and three volumes of oil of cassia. This has a refracting index of
about 1.54. The mixture, after being shaken up very thoroughly, should be allowed
to settle & the clear liquid used.
.
It should be observed that the borosilicate flint glass will tarnish badly if
allowed to lie about exposed to the impure air of the workshop for many days.

page_text: .
& should consequently be placed in a tin box when not actually required for
testing.
It may be often advisable after the objective has been properly & exactly
centered into its final cell, to repolish over both sides of the borosilicate flint as a
final finish.
If all the surfaces appear to be well & regularly figured & yet there is a little
residual spherical aberration visible at the focus, (doubtless caused by some surface
being rather parabolic) the edge rays focussing short, then the brass ring may be
thinned down somewhat until the aberration is perfect.
After that condition has been secured then nothing remains but to alter the
radius of the 6th surface in the usual manner, if necessary, until the colour
correction is judged to be perfect.
In giving tables setting forth the almost perfect achromatism of my objective I
have assumed that for the positive lens L₁ is equal to
lens La
P1
1
Ps
for the other positive
This assumption happens to fall in remarkably well with the optical properties
of the two glasses and also makes the calculations simple for illustrative purposes.
But, it should here be pointed out that this relation of equality can be departed
from within narrow limits without appreciable prejudice to the perfection of the
objective's colour correction. For instance, in the case of the curves above
1
specified for the first lens
14
-
1
about 97 for the 3rd lens. But if
Ps
1
P1
'were less
in value than about ths of 1, then an appreciable amount of secondary colour
15
P8
would be noticeable at the focus of the objective, if a large one.
I should also point out that the optical properties of the three sorts of glass
herein specified are not rigidly fixed, but small variations in their optical properties
occur in the ordinary course of manufacture, and glasses are often turned out by
the manufacturers which are intended to correspond to a certain catalogued type,
but which actually turn out about intermediate in optical properties between the
type of glass intended to be copied and the next closely analogous glass.
For instance I have seen Messrs. Schott's statement to the effect that the light
silicate crown glass 0-374, herein specified, differs from the next catalogued
type 0.337 only in having a slightly larger proportion of alkali in its composition.
The two types differ so very slightly & so insensibly run into one another in
different meltings, that they should more appropriately be described as minor
variations of the same type. Nevertheless, this glass is most perfectly suited to
my purposes, so far as I can yet judge, when it contains such a quantity of alkali
as will raise its dispersive power a mere trifle higher than that herein specified and
given in their catalogue, thus tending to differentiate it a trifle more from the type
named 0.337, which latter is not so suitable, inasmuch as its employment would
give rise to perceptible secondary colour aberrations & throw the photographic blue
rays somewhat beyond the visual focus.
Hence after further trials with larger objectives I may feel justified in inducing
Messrs. Schott or other manufacturers of this glass, should they exist, to slightly
increase the proportion of alkali in this glass, without prejudice to its reasonably
agreeing in its optical properties with those specified herein, and without any
necessity for materially altering the curves &c. herein specified.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what
I claim is
1. The triple combination of two positive lenses, one of the baryta light flint
glass and the other of the light silicate crown glass, with a negative lens of the

page_text: boro-silicate flint glass, the three glasses severally having the optical properties
herein specified, within reasonable limits of variation; and the three lenses being
so adjusted in power, substantially as herein specified, but irrespective of their
order, as to yield an object-glass giving an image practically quite free from those
secondary colour aberrations which are present in the image yielded by objectives
of ordinary construction.
2. The special disposition of lenses and combination of curves, substantially as
herein specified, and as applying to the above defined combination; which curves
render it possible to test all surfaces for defective figuring, secure that the objective
is not only free from spherical aberration in a general sense, but for all colours
simultaneously, and that the largest possible field of good definition is secured, as
well as other advantages.
Dated thie 5th dox of July 1902
H. DENNIS TAYLOR.