66 BACTERIOLOGY. 



the sine of the angle of incidence of a ray passing from a vacuum 

 into glass being to the sine of the index of refraction as 1'6 to 1. 



If we study the course of a pencil of rays we find that some 

 of the rays are reflected instead of entering the medium and being 

 refracted. When, for example, a pencil of rays falls upon water 

 or glass, after passing through air, some of the rays are lost by 

 reflection, and the proportion of the lost rays will increase with 

 their obliquity. The diminution of the brightness of the image 

 when pencils of rays have to pass through lenses is thus accounted 

 for, and this loss of light increases when the number of surfaces 



-B 



FIG. 11. THE REFRACTION OF LIGHT. 



through which the rays pass are, as in high-power objectives,. 

 increased. There is an additional loss when there is an increase in 

 the difference between the refractive power of the different media 

 through which light passes. When pencils of rays pass from glass 

 into air, and then into glass again, the loss is much greater than 

 when the air is replaced by a medium with a refractive index mor& 

 nearly approaching that of glass. This explains the value of tha 

 immersion system, which will be referred to more fully later on, 

 and also the advantage of cementing pairs of lenses with Canada 

 balsam or glass paste. The lenses used in the optical arrangements 



