138 A MANUAL OF BACTERIOLOGY 



proportion) let s represent an ordinary glass micro-slide, x a layer 

 of Canada balsam in which the object is mounted, and covered 

 with the cover-glass G, while L is the objective with its front lens. 

 Let the object be illuminated by the ray of light Yy ; this on enter- 

 ing the glass of the slide and the Canada balsam will be refracted 

 or bent nearer the perpendicular and will proceed in the direction 

 yt. Canada balsam, and also cedar oil, produce about the same 

 amount of " refraction," or bending of a ray of light, as crown 

 glass, and hence these three substances crown glass, Canada 

 balsam, cedar oil are said to have the same " refractive index," 

 and, consequently, the glass of the slide, the Canada balsam, and 

 the cover-glass act as one homogeneous medium, and the line yt 

 is a straight one. In the first place, let us suppose that the 

 objective L is a dry one, having a layer of air between its front lens 

 and the cover-glass ; then the ray of light, on emerging from the 

 cover-glass into the air, is now bent away from the perpendicular 

 and pursues a direction practically parallel to its former one, 

 represented by the line tw, and misses the lens altogether the 

 lens is unable to take it up. If, however, we suppose that our 

 objective is an oil-immersion one, and that a drop of cedar oil 

 takes the place of the layer of air between the cover-glass and the 

 front lens in the foregoing example, then the glass slide, Canada 

 balsam, cover-glass, cedar oil, and the front lens of the objective 

 form practically one medium ; they all have the same refractive 

 index and produce the same amount of refraction or bending of a 

 ray of light. Therefore the direction of the ray forms a straight 

 line in all these, and the ray passes into the objective as is repre- 

 sented by the broken line t v. More important still, however, is 

 that which happens to rays which fall on the slide at a very oblique 

 angle. In the same figure (Fig. 25) let ef represent such a ray ; 

 on entering the slide it will be refracted, and its passage through 

 the slide, balsam, and cover-glass may be represented by fk. As 

 before, let us suppose that in the first place our objective is a dry 

 one, and that we have a layer of air between the cover-glass and its 

 front lens. In this case, if the angle which fk makes with the 

 perpendicular is greater than about 39 or 40, the ray, instead of 

 emerging from the cover-glass into the layer of air, is totally reflected 

 by the cover-glass and pursues a course roughly represented by kr, 

 so that it never enters the objective. If, however, we employ an 

 oil-immersion objective, with oil instead of air between the cover- 

 glass and its front lens, then, as before, the slide, balsam, cover- 

 glass, oil, and front lens of the objective form practically one 

 homogeneous whole, and the ray efk, instead of being totally 



