THE MICROSCOPE AND ITS REVELATIONS. 



exactly as 1^- to 1, or as 4 to 3. So, again, the index of refraction for 

 (flint) Glass, being about 1.6, we mean that the sine e e' of the angle of 

 incidence of a ray E o c passing into Glass from a vacuum, is to the sine 

 of (j g' the angle of refraction G o c', as 1.6 to 1, or as 8 to 5. So in the 

 Fia.'i. case of Diamond, the sine 



e e' is to the sine d d' as 

 2.439 to 1, or almost ex- 

 actly as 2^- to 1, or as 5 

 to 2. Thus, the angle of 

 incidence being given, the 

 angle of refraction may 

 be always found by divid- 

 ing the sine of the former 

 by the 'index of refrac- 

 tion/ which will give the 

 sine of the latter. In ac- 

 cordance with these laws, 

 a ray of light passing from 

 one medium to another 

 perpendicularly to the 

 surface which divides 

 them undergoes no re- 

 fraction; and of several 

 rays entering at different- 

 angles, those nearer the perpendicular are refracted less than those more- 

 inclined to the refracting surface. When a pencil of rays, however, im- 

 pinges on the surface of a denser medium (as when rays passing throughi 

 Air fall upon "Water or Glass), some of the incident rays are reflected from, 

 that surface, instead of entering it and undergoing refraction; and the pro- 

 portion of these rays increases with the increase of their obliquity. Hence- 

 there is a loss of light in every case in which pencils of rays are made to 

 pass through lenses or prisms: and this diminution in the brightness of 

 the image formed by refraction will bear a proportion, on the one hand, 

 to the number of surfaces through which the rays have had to pass; and, 

 on the other, to the degree of obliquity of the incident rays, and to the- 

 difference of the refractive powers of the two media. Hence, in the 

 assage of a pencil of rays out of Glass into Air, and then from Air into 

 again, the loss of light is much greater than it is when some 

 medium of higher refractive power than air is interposed between the two- 

 glass surfaces; and advantage is taken of this principle in the construc- 

 tion of Achromatic objectives for the Microscope, the component lenses 

 of each pair or triplet ( 14) being cemented together by Canada Balsam ^ 

 as also in the interposition of Water or some other liquid between the 

 covering-glass of the object and the front lens of the objective, in the 

 ' immersion lenses' now coming into general use ( 19). On the other 

 hand, advantage is taken of the partial reflection of rays passing from air 

 into glass at an oblique angle to the surface of the latter, in the construc- 

 tion of the ingenious (non-stereoscopic) Binoculars of Messrs. Powell and 

 Lealand and of Mr. Wenham ( 81). 



2. When, on the other hand, a ray, w o, emerges from a dense 

 medium into a rare one, instead of following the straight course, it is 

 bent from the perpendicular according to the same ratio; and to find the 

 course of the emergent ray, the sine of the angle of incidence must be 

 multiplied by the ' index of refraction/ which will give the sine of the 



