22 MICROSCOPE AND ACCESSORIES. 



mirror and oblique light. In order to appreciate the difference between 

 this dark ground and ordinary transmitted light illumination, use an 

 ordinary diaphragm and observe the crystals. 



A very striking and instructive experiment may be made by adding 

 a very small drop of the solution to the dried preparation, putting it 

 under the microscope very quickly, lighting for dark-ground illumina- 

 tion and then watching the crystallization. 



REFRACTION AND COEOR IMAGES. 



\ 61. Refraction Images are those mostly seen in studying microscopic objects. 

 They are the appearances due to the refraction of the light in passing from the 

 mounting medium into the object and from the object back into the mounting 

 medium. With such images the diaphragms should not be too large (see \ 45). 



If the color and refractive index of the object were exactly like the mount- 

 ing medium it could not be seen. In most cases both refractive index and color 

 differ somewhat, there is then a combination of color and refraction images which 

 is a great advantage. This method of illumination is mostly used in histology. 



\ 62. Color Images. — These are images of objects which are strongly colored 

 and lighted with so wide an aperture that the refraction images are drowned in 

 the light. Such images are obtained by removing the diaphragm or by using a 

 larger opening. This method of illumination is specially applicable to the study 

 of stained microbes. (See below \ 67). 



ADJUSTABLE, WATER AND HOMOGENEOUS OBJECTIVES. 



EXPERIMENTS. 



§ 63. Adjustment for Objectives.— As stated above (§ 16), the ab- 

 erration produced by the cover-glass (PI. V., Fig. 41), is compensated 

 for by giving the combinations in the objective a different relative posi- 

 tion than they would have if the objective were to be used on uncovered 

 objects. Although this relative position cannot be changed in unad- 

 justable objectives, one can secure the best results of which the object- 

 ive is capable by selecting covers of the thickness for which the object- 

 ive was corrected. (See table in § 17). Adjustment may be made 

 also by increasing the tube -length (§17) for covers thinner 'than the stand- 

 ard, and by shortening the tube-length for covers thicker than the 

 standard (§ 17). 



Adjustable Objectives. — The proper adjustment of objectives, that is, 

 the adjustment which gives the truest image, requires both insight and 

 experience ; for the structure of an object does not appear the same 

 with different adjustments of the objective. And as the opinion of 

 different observers on the structure of objects varies, they adjust the 

 objectives differently, and try to obtain the adjustment which will show 

 a structure in accordance with their opinion. Kyes also differ, and two 

 observers might find it necessary to adjust the same objective differently 

 to produce an identical appearance for each of them. 



