568 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 108. 



it is always seen under the same angle, and corresponds to a retinal 

 image of the same size. 



Let us now suppose the image to be received at the distance of 

 distinct vision, and let us see what, in these conditions, are the rela- 

 tions which exist between the 

 dimensions of the object A B 

 and those of the drawing 

 a h'. 



Observe in the first place 

 that A B and a' b' form the 

 base of two similar isosceles 

 triangles, A B and a'h' ; 

 but in order that the bases 

 may be equal, it is necessary 

 that the altitudes should be 

 so also, tliat is, that the dis- 

 tance of the drawing from the 

 eye should be equal to that of 

 the eye from the object. Is 

 this so in practice ? Cer- 

 tainly not, or at least very 

 exceptionally. Indeed, if the 

 distance of the drawing from 

 the eye is constant (the dis- 

 tance of distinct vision), that 

 of the eye from the object is 

 very variable. It varies ac- 

 cording as the tube of the 

 Microscope is more or less 

 raised above the stage, and 

 thus varying it may be either 

 greater or less than that of 

 distinct vision. 



If the distance of the 

 drawing from the eye is 

 greater than that of the eye 

 from the object, which is the usual case wdth our ordinary Micro- 

 scopes with short tubes, the drawing a' V will be necessarily larger 

 than the object A B, If it is less, on the contrary, the drawing will 

 be smaller. 



A calculation may serve to give an idea of the extent of these 

 variations. Let us suppose an object of 10 mm. in diameter at a dis- 

 tance of 20 cm. from the eye. Take for the distance of distinct 

 vision 10 Paris inches, say, 27 cm., and calculate what will be the 

 diameter of the drawing. In the similar triangles A B and a' h' 

 the bases being proportional to the altitude, we shall have 



x_ _ 27 

 10~ 20' 



:r = 13'5 mm. 



Suppose, on the contrary, thp.t the distance of the eye from the 



