A New or Improved Optical Instrument for the Detection of Imitation Diamonds.
Year: 1894

page_text: Date of Application, 6th Oct., 1894
Complete Specification Left, 6th July, 1895-Accepted, 17th Aug, 1895
PROVISIONAL SPECIFICATION.
A New or Improved Optical Instrument for the Detection of
Imitation Diamonds.
I, THOMAS THORP of Whitefield near Manchester in the County of Lancaster
Architect and Civil Engineer do hereby declare the nature of this invention to be
as follows
This invention consists in the construction of an optical apparatus or instrument
having for its object the detection of imitation diamonds o "paste" when set as
cut brilliants.
The construction of the said apparatus or instrument is based upon the following
optical principles
It is well known among physicists that when light passes from one refractive
medium to another possessing greater refractive properties, some of the light
incident upon the latter medium is reflected and renders the form of the reflecting
surfaces visible.
In the case however where light passes from a higher to a lower refractive
medium there is an angle from the normal, known as the "critical angle," beyond
which no light is transmitted but is all reflected into the higher refractive medium,
and puems of glass made beyond the "critical angle" are known as "totally
reflecting prisms.
"
Now, as the refractive properties of the diamond very greatly exceed those of
any other known crystal or other refractive medium, a considerable amount of light
1s reflected from its sui face, or in other words, it remains quite visible, even when
immersed in, or looked at, through a medium which endeis ordinary crystals or
paste invisible or nearly so.
In caying out my invention I make use of a reflecting pism, a totally reflecting
one being the most efficient and preferably formed by cutting one end of a short
glass rod, diagonally at an angle of about 45° and having a lenticular surface
formed at the other end, the focus of the lens being at or about the centre of the
reflecting surface. The portion of the glass rod round the end cut diagonally 13
preferably ground dull so as to diffuse the light falling upon the reflecting surface
The piism and lens may however be made separately and attached to opposite
ends of a suitable tube, or the prism aloue may be used when the crystals to be
tested are sufficiently large as not to require magnification.
In order to test a crystal with an apparatus of insti ument constructed according
to this invention, one of its facets is placed directly against the polished diagonal
surface, moistened previously with a little liquid, preferably one having a high
refractive index; but water will suffice when the user is accustomed to the
apparatus.
In case the crystal is a diamond, considerable light will be seen reflected from it
by viewing it through the lenticular end of the instrument and the facet will be
very distinct; whereas if the crystal is "paste" the lacet will be scarcely to be
distinguished from the sun
11ounding medium in which it is immersed
Dated this 5th day of October 1894.
THOMAS THORT,
Applicant.

page_text: A New or Improved Optical Instrument for the Detection of
Imitation Diamonds.
I, THOMAS THORP, Whitefield near Manchester in the County of Lancaster,
Architect and Civil Engineer, 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
This invention consists in the construction of an optical instrument having for
its object the testing of cut diamonds and the detection of imitation diamonds or
"paste" when cut and set as bulliauts
The construction of the said instrument is based upon the following optical
principles
-
As is well known amongst physicists, when light passes from one refractive
medium to another possessing greater refractive properties, some of the light
incident upon the latter medium is reflected into the former and renders the form
of the reflecting surfaces visible when looked at through the former medium.
Where light passes from one medium to another having the same refractive
power, no light is reflected but all is transmitted through the second medium, which
is thus invisible when looked at through the first.
In the case where light passes from a ingher to a lower refractive medium there
1s an angle from the normal known as the critical angle, beyond which no light is
transmitted but all is reflected into the higher medium, and prisms of glass whose
angle exceeds the critical angle are known as totally reflecting prisms.
Now as the refractive power of the diamond greatly exceeds that of any other
known crystal or medium used in imitation of the diamond, a considerable amount
of light is reflected from its surface, so that it remains quite visible wheu immersed
in or looked at through a medium which renders ordinary crystals or paste invisible
or nearly so.
The improved instrument based upon the principles stated above is shewn on the
drawings in two modified forms of construction
Fig. 1 shewing an end view and Fig. 2 a sectional side view of the same.
Figs. 3 and 4 similar views of a modified form thereof
In carrying out my invention I make use of a reflecting prism, a totally reflecting
one being the most efficient, which prism a is preferably formed by cutting one end
of a short glass rod b at an angle of about 45 degrees to the axis, and grinding the
sides a and face a flat, though this is not indispensable. The reflecting surface a³
1s polished and the sides and face of the prisin or the glass rod 10und the end cut
at an angle are preferably ground dull go as to diffuse the light falling upon the
icflecting surface a³, and the glass iod beyond the prisin is coated with black or
enclosed in a tube to exclude other light. At the other end of the glass rod a
lenticular surface d may be formed, the focus of the lens being at or about the
centre of the reflecting sui face a³, or instead thereof a lens d'Fig. 4 may be hxed
in the end of a tube e which may slide in the tube cinto which the glass rod b
which in this case is made shorter and cut square at the end, is inserted, so that
the surface a can be accurately focussed.
The object of the lens or lenticular surface 15 to magnify the crystals inspected
through the instrument and the end of the glass rod may be cut and polished flat or
square to the axis if the crystals to be tested are suficiently large not to require
magnification.
In order to test a crystal f with an instrument constructed as herein before
described, one of its facets is placed duectly against the polished inclined surface a³
moistened previously with a little liquid preferably with one having a high refractive

page_text: index, such as cedar or cassia oil, but water will suffice when the user is accustomed
to the lustrament.
In case the crystal is a diamond, considerable light will be seen reflected from
the facet touching the prism and when viewing it through the lenticular end of the
instrument the facet will be very distinctly seen showing brightly with sharp
outlines, whereas if the crystal is paste or other substance used in imitation of
diamonds, the facet will be scarcely distinguishable from the surrounding medium
in which it is immersed and appear without any distinct outlines.
Having now particularly described and ascertained the nature of my said inven-
tion and in what maunei the same is to be performed, I declare that what I
claim is.
1. The construction and use of an optical instrument for testing diamonds
substantially as herein before described with reference to Figures 1 and 2 of the
drawing.
2. The construction and use of an optical instrument for testing diamonds
substantially as herein before described with reference to Figures 3 and 4 of the
drawings.
3. In a diamond testing instrument the use of a prism in the manner and for the
purpose hereinbefore described
rpose hereinbefore described
Dated the 4th day of July 1905
R. J. URQUHART,
57, Barton Arcade, Manchester, and 5, Clayton Square, Liverpool,
Consulting Engineer and Patent Agent.
R. J. URQUHART,
57, Barton Arcade, Manchester, and 5, Clayton Square, Liverpool,
Consulting Engineer and Patent Agent.