CHAPTER V. 



GENERAL REMARKS UPON OBJECTIVES TEST OBJECTS. 



WHEN we consider the many adjustments of apparatus 

 needed ere a correct picture of an object can be placed 

 before the eye, it will be readily seen how necessary it is to 

 pay strict attention to details more especially of illumina- 

 tion, this being one of the first and most important lessons 

 the microscopist has to learn. 



When rays of light pass through media with parallel 

 faces, such as the glass slips used by every worker with the 

 microscope, the emerging rays are parallel with those enter- 

 ing, the intermediate portion being bent away from both 

 these planes as shown in Fig. 86. If water be used above 

 the glass, the emergent ray will be bent up more towards 

 the perpendicular, while when cedar-wood oil, or any of the 

 homogeneous-immersion fluids are employed, the path of 

 the ray will be one continuous 

 line from the under side of the 

 glass slip. A diagram of the 

 passage of a light-ray through 

 glass is shown in Fig. 86. 



Ordinary glass slides, and also 



the thin covers, are made from 



FIG. 86. 

 crown glass having a refractive 



index varying from 1-5 to 1*525 referred to air as unity: 

 the following table of mean refractive indices of many sub- 

 stances used by the microscopist may not be uninter- 

 esting : 



H 



