4 GORHAM, ON THE 



opaque spot or streak which occupies exactly the same posi- 

 tion in each. Similar results are obtained if a small dot, no 

 big-ger than a pin's head, be made on a slip of glass with 

 Indian ink, and introduced into the eye-piece of the diascope. 

 The dot now appears multiplied, and as many images of it are 

 seen as there are apertures by which it is made visible. 



In like manner if a small semicircle be painted on glass, its 

 images will be multiplied, but each image will be seen in- 

 verted, and will appear as a black body on an illuminated and 

 coloured ground. 



And if a small triangle, or any other figure of definite 

 shape, be cut from a piece of black paper, and if the opening 

 thus made be examined in the same way, a number of illumi- 

 nated and coloured triangles will be seen on a black ground. 



It is when transparent figures are made according to this last 

 method that really beautiful combinations may be produced. 

 But here, in order to insure success, it is necessary that the 

 transparent openings should never exceed in size the pupillary 

 aperture of the eye, fig. 16. Hence they should always be 

 made within the limits of a circle, the 0'18 of an inch in 

 diameter, fig. 15 ; this being the mean of the greatest and least 



Fig. 15. Fig. 16. 



o 



expansion of the pupil. If such transparent figures are made 

 greater than this, the margin of the pupil will obstruct some 

 of the external rays, and the outline of the image will thus be 

 lost or badly defined. If, on the other hand, they are less, the 

 quantity of light admitted into the eye will be too small, and 

 the images but feebly illuminated. 



Such openings used as objects are to be considered as little 

 else than artificial pupils, modifying the shape and contracting 

 the size of every cone of light which is admitted into the eye 

 from small apertures. 



When one of these transparent openings is held close to the 

 eye, and examined with common diffused light, it becomes 

 altogether invisible ; but, when it is viewed by the aid of a 

 pencil of light from a small inlet, its outline is well defined 

 and much magnified ; and the disc of the inlet, which would 

 otherwise be circular, is replaced by the pattern we may 

 choose to give to the transparency. 



And if the pupil of the eye itself, obliterated, as it often 

 is, from disease, have a small portion excised from it, as in 



