32 GORHAM, ON THE MAGNIFYING 



close in front of the eye, and a small divergent pencil of light. 

 From which it results, that the object when held in this pencil, 

 intercepts a portion of the light, and so casts a shadoxo greater 

 than itself which shadow is rendered visible by contrast, still 

 further magnified by proximity, and eventually forms a visible 

 image at the bottom of the eye. 



This principle of opposition or dissimilitude of shade, as 

 well as of colouring, called contrast, a term in very general use 

 in painting, is of universal application, because it contributes 

 not to the beauty only, but to the visibility of all objects. 

 Whether these opposite and contiguous colours or shades are 

 seen at the same time, and that this gives rise to the effect of 

 which we are all sensible, as is generally supposed, or whether 

 it results from attentively looking at the one and then at the 

 other in rapid succession, as was insisted upon by Sir Charles 

 Bell, it is not our province now to inquire, although there are 

 reasons for believing that both of these theories are correct, 

 and that the former holds good for minute objects near to the 

 eye, while the latter applies to larger ones at greater or com- 

 mon intervals. Dismissing hypothesis, however, we know that 

 with respect to bodies viewed at ordinary distances, if a 

 white figure be delineated on a white ground, or a black figure 

 on a black ground, neither is visible ; in the first there is no 

 shade, and in the second no light, consequently there is no con- 

 trast But the slightest variation of shade in the figures in 

 relation to their respective grounds, is sufficient to render each 

 of them definite. Hence the effect of a well-executed en- 

 graving, in which, although no colour is introduced, but merely 

 white and black to imitate light and shade, the appearance is 

 natural and satisfactory. 



Two simple experiments will serve to show the importance 

 of attending to contrast with respect to the examination of 

 very near objects. By the first it is seen that although a well- 

 defined image is hioicn to be certainly received on the retina, it 

 is invisible ivhen the retina and it happen to be equally illumi- 

 nated. For this purpose, let perforations with a needle, the 

 tenth of an inch apart, and arranged in the form of a circle 

 of about a quarter of an inch in diameter, be made in a piece 

 of blackened cardboard (fig. 3). When brought close to the 

 eye, these apertures appear as a ring of luminous circles (fig. 

 4), the remaining part of the retina being in darkness. If now 

 a round piece be cut out from the centre of the first card, a 

 portion as large, for instance, as that which is traced in outline, 

 but not actually excised in figure 3, so as to admit light 

 through the very middle of the perforated circle ; it will be 

 found that while the discs are known to be still received on the 



