OF THE mCKOSCOPE. 79 



the cover ; o the object, and o o' a ray of light proceeding 

 from that object. It is obvious that when the space, E, is 

 filled with air, the ray, o o', after passing through the cover- 

 glass and suffering refraction within it, will, on emerging, pass 

 on in a direction parallel to its original path, and so will pursue 

 the line, I d, and pass outside of the objective. But if the 

 space, E, be filled with water, it will take the course I c, and so 

 will enter the front lens. 



It is evident that if a fluid of greater refracting power than 

 water were used, the result would be still more marked. This 

 consideration led to the use of glycerine instead of water, with 

 notably good results, and finally lenses have been constructed 

 in which a liquid of the same refractive and dispersive power, 

 as the glass of the thin cover, is used, and such lenses are known 

 as "objectives of homogeneous immersion," or, for short, 

 "homogeneous objectives" the word homogeneous meaning of 

 the same kind, and applying to the liquid and the cover-glass. 



An immerson fluid, perfectly homogeneous with the cover- 

 glass, would evidently obviate all necessity for what is known 

 as "cover correction," provided certain other conditions in the 

 mounting of the object and the construction of the microscope 

 always remained the same, and most of the objectives made by 

 Zeiss on the homogeneous system have no such correction. 

 But since a difference in the quality or thickness of the me- 

 dium in which the object is mounted, or a difference in the 

 length of the tube or body of the microscope used with 

 the objective, gives rise to serious disturbances, American 

 makers have preferred to make their best homogeneous ob- 

 jectives "adjustable," as it is called ; that is, provided with the 

 visual means for cover correction. 



It is a general impression that immersion objectives are more 

 difficult to use than dry ones ; such is not the case, however. 

 Immersion objectives, with the same degree of effort and skill, 

 give finer results than dry objectives of the same power, though 

 of course for the ordinary rapid work of searching for known 

 objects, dry lenses are to be preferred as being less trouble- 

 some, for it is undeniable that it is some trouble to apply the 

 liquid and clean the lens, and also the cover, and where a large 

 number of observations are to be made, even trifling delays 



