THE MICROSCOPE. 171 



Some mordants act by penetrating the substance of the bacte- 

 ria and assisting the entrance of the dye ; and so produce a very 

 intense staining which will resist the subsequent washings, etc. 

 Others act by saturating the bacteria and producing therein a 

 difficulty soluble prec pitate with the dye. 



173. Is there a difference between homogeneous and oil immersion 

 objectives ? Can prepared cedar oil be used, with a homogeneous im- 

 mersion objective f — Enquirer. 



It is always best to use the immersion fluid recommended by 

 the maker of the objective ; but, as cedar oil and homogeneous 

 immersion fluid are usually prepared so as to have the same 

 refractive index as the front lens of the objective (optical glass), 

 either fluid may be used, as a rule. 



174. Is aniline water the same thing as aniline oil ? If not, how 

 is aniline water prepared f — Enquirer. 



Three drops of aniline oil to a dram of water, well shaken to- 

 gether and filtered, is called aniline water. A solution of a dye 

 in aniline water will keep good about one week. 



PRACTICAL SUGGESTIONS. 



By L. A. WILLSON, 



CLEVELAND, OHIO. 



Collar Correction of Objectives. — The usual instruction 

 given to a novice for the correction of his adjustable objective 

 is to turn the collar until a clear and distinct image is obtained 

 This is vague, indefinite, and misleading, because it is almost 

 impossible to know when a perfecty clear and distinct image of 

 an unknown object has been obtained. A more certain, cor- 

 rect, and scientific test is by color. If noticed carefully, all ob- 

 jectives show some of the colors of the spectrum in the speci- 

 mens viewed and this can be taken advantage of for the correc- 

 tion of the lens. The following table will solve the problem : 



CENTRAL LIGHT. 



When under-corrected — elevations red. 

 When under-corrected — depressions green. 

 When just right — elevations light violet. 

 When just right — depressions green piprjin. 



