Slide viewer or projector

A slide viewer or projector is provided with a device for preventing two or more slides from attempting to enter into the viewing or projecting area simultaneously. The device comprises a barrier with a wiper pivotally mounted thereon. The barrier and wiper are spaced apart from the path taken by a slide so that the slide pivots the wiper against the action of biasing means and then the barrier is raised into engagement with the upper surface of the slide and this allows the wiper to wipe over the upper surface of the slide to "kick off" any other slide resting on the surface.

FIELD OF THE INVENTION 
This invention relates to apparatus for viewing or projecting photographic 
transparencies, hereinafter referred to as "slides". In particular, the 
invention is concerned with slide viewing or projecting apparatus which 
has provision for displacing slides one at a time from a stack of slides 
to a viewing or projecting area. 
With such viewers and projectors, there has up to now been a problem in 
feeding the slides one at a time from the stack to the viewing or 
projecting area. This problem is due to the tendency for more than one 
slide to be displaced simultaneously into the viewing or projecting area 
and this results in a blocking of the path leading to the viewing or 
projecting area. 
Slides are not of a uniform thickness and it is difficult therefore to 
design a slide changer, whether it be manually or automatically operable, 
which accommodates one, and only one, slide at a time. Furthermore, slides 
become distorted and damaged after a period of use and, in such a 
condition, one slide may catch or grip a section of another slide and a 
slide which is displaced towards the viewing or projecting area may drag 
an adjacent slide with it. Even slides that are not distorted can exert a 
drag upon each other since they are not particularly smooth and this again 
creates the possibility of two slides attempting to enter into the viewing 
or projecting area simultaneously. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a slide viewer or 
projector having means by which this difficulty is overcome. 
According to the present invention, a slide viewer or projector has a 
region for receiving a stack of slides and means providing a path 
extending from said region to a viewing or projecting area, a slide 
changer for displacing slides one at a time along said path from the 
region to said area, a barrier spaced from the path by a distance less 
than the thickness of the thinnest slide to be accommodated, a wiper 
pivotally mounted on the barrier and biased to a position in which a 
portion of the wiper is spaced from the path by a distance not less than 
the separation of the barrier from the path and is positioned between the 
barrier and the region, the action of a slide moving from said region to 
said area causing the wiper to be pivoted against the action of the 
biasing means until the barrier is raised on to the upper surface of the 
slide and the wiper then pivots back and wipes over the upper surface of 
the slide. 
The action of the wiper wiping over the upper surface of the slide ensures 
that a second slide cannot be displaced with the first-mentioned slide due 
to the second slide being caught on the upper surface of the 
first-mentioned slide. 
In a preferred arrangement, the wiper comprises a plate including a tip 
which is engageable by a slide moving along said path and the biasing 
means comprise a leaf spring engaging one edge of the plate. If desired, a 
pair of wipers each as described above may be employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a slide viewer comprises a casing 1, conveniently 
of plastics material, supporting a lens structure 3 and a viewing screen 
5. An electric lamp (not shown) is positioned in the casing 1 and the lamp 
is energised either from a number of electric batteries (not shown) or 
from a mains supply. Slides 6 to be viewed are arranged in a stack in a 
region 7 which is to one side of the lens structure. A semi-automatic 
slide changing device, indicated generally by reference 9, is provided for 
feeding the slides one at a time from the bottom of the stack into a 
viewing area 11 positioned between the lamp and the lens structure and for 
withdrawing the slides, after they have been viewed, from the viewing area 
to a second stacking area 13 which is beneath the region 7. The slide 
changing device includes a carrier platform 15 which provides a path from 
the stacking region 7 to the viewing area 11. The platform has an 
upstanding projection referred to as a pusher which, when the platform is 
moved to the right to its fullest extent, lies to the right of the lowest 
slide in the region 7. On moving the platform to the left to the position 
shown in FIG. 1, the pusher engages the lowest slide and pushes it into 
position above the viewing area. When the platform is next moved to the 
right, the slide is prevented from moving with it but the previously 
viewed slide is withdrawn from the viewing area to the area 13. The new 
slide then drops into the viewing area. 
Between the region and the viewing area, and associated with the path 
therebetween, there is a barrier 17 which is spaced from the path by a 
distance less than the thickness of the thinnest slide to be accommodated. 
A wiper 19 is pivotally mounted on the barrier and the barrier and the 
wiper together serve to prevent any tendency for more than one slide to 
move at a time along the path into the viewing area. The construction of 
the barrier and wiper is shown in more detail in FIGS. 2, 3 and 4. 
Referring to FIGS. 2, 3 and 4, it can be seen that the barrier comprises a 
member extending across the width of the path 21 with a bulged portion 23 
of the barrier being spaced from the path 21 by a distance which is less 
than the thinnest slide 25 which is to be accommodated. The wiper 19 
comprises a plate-like member pivoted at 27 to the barrier and having a 
tip 29 which is positioned between the bulge 23 and the region 7 and which 
is separated from the path 21 by a distance which is not less than the 
separation of the bulge 23 from the path 21 and which is such as to be 
encountered by a slide moving along the path 21. The wiper has a recessed 
portion 31 into which one end of a leaf spring 33 projects. This leaf 
spring provides a degree of biasing to the wiper and, in its stable 
position, the wiper is positioned relative to the barrier, as shown in 
FIG. 2. 
When a slide 25 is moved by the slide changer from the stacking region 7 to 
the viewing region, it first encounters the tip 29 of the wiper causing 
the wiper to be pivoted against the action of the spring 33 to the 
position shown in FIG. 3. At this position, the lower end of the tip is in 
engagement with the upper surface of the slide 25 but the bulge 23 
projects below the upper surface of the slide. Further movement of the 
slide towards the viewing area causes the slide to abut against the bulge 
23 to lift the barrier sufficiently for the bulge to move along the upper 
surface of the slide. The tip of the wiper is then freed from contact with 
the upper surface of the slide and the leaf spring 33 causes the wiper to 
pivot in an anti-clockwise direction and the tip moves over the upper 
surface of the slide and, if a second slide resting on the upper surface 
of the lower slide is attempting to enter into the viewing area, then the 
wiping action of the wiper kicks the upper slide back into the stacking 
region 7. After the wiper has moved across the upper surface of the slide, 
it returns to its initial position, as shown in FIG. 4. 
Although only one wiper has been described, it is convenient for a pair of 
wipers to be provided, each being in the form of a plate and each 
pivotally mounted at an opposite end of the barrier. Each wiper is engaged 
by a leaf spring to provide a bias for the wiper. 
The leaf spring may be formed integral with the barrier 17. 
Although a slide viewer has been described, a slide projector may also be 
provided with the barrier and wiper, as described above, to prevent more 
than one slide attempting to enter from a stack of slides into the 
projecting area.