MAGNIFYING POWER OF SHORT SPACES, 3 



which revolves in the groove, G, and to this rim is attached 

 a second rim, r, which is provided with a female screw, and 

 this secures any circular plane of apertures, p, we may choose 

 to insert, at the same time that it admits of its revolution. 



A series of objects, either painted or mounted on glass slides, 

 can thus be examined at one end by the light admitted through 

 perforated planes at the other. As such an optical contrivance 

 is a mere conductor of the light directly from the apertures to 

 the eye, while at the same time it excludes all the extraneous 

 rays, it may not inaptly and for convenience' sake be denomi- 

 nated a diascope : a term derived from the two Greek words, 

 liac, through, and axoirsco, I view. And when it is usetl for 

 the purpose of multiplying images, it may be called the mul- 

 tij?lying diascope. 



The circular planes containing the apertures are made of 

 thick pasteboard, perforated with a needle at intervals of the 

 one-tenth of an inch as before stated ; and the openings may 

 be arranged in a variety of combinations, as shown in the 

 patterns from one to six, page 225, of the last paper. 



Here let us notice that the round form of the images, de- 

 picted on the retina when examining small apertures in this 

 way, is determined by the shape of the pupillary opening of 

 the eye rather than by that of the apertures themselves. 

 Hence it is always circular whether these be round, triangular^ 

 square, or altogether irregular in outline. This can easily be 

 proved by perforating cards with triangular or square needles. 

 The openings thus made when brought very near to the eye 

 always appear circular. A little reflection will show that this 

 is a necessary result, inasmucli as the outermost rays of the 

 rapidly-diverging cones are intercepted by the iris, while the 

 more internal rays pass on through the pupil, thus receiving 

 their circular form. 



Hence if the pupil be widely expanded the discs will be 

 large, and vice versa, but nevertheless always perfectly round. 



Let us now take a circular plane, presenting a combination 

 of perforations arranged as to colour and relative position as 

 in the outline, No. 4, page 225, of the last paper ; and let 

 us notice what kinds of images are presented to the eye when 

 small bodies are examined by the light transmitted through 

 them. 



Holding the instrument to the eye with a view to examine 

 the apertures, we observe that the smallest particle of dust, or 

 film of mucus, happening to exist on the surface of the 

 transparent cornea, is immediately detected ; and although 

 such bodies have no definite shape, they are to be recognised 

 interfering with the transparency of the discs, and forming an 



B 2 



