THE MICROSCOPE. 



justed so as to clearly define an object without a cover-glass, 

 it will fail to do so if it have a cover. Mr. Ross first pointed 

 this out, and made the front lens of the objective movable, so 

 that the observer might adjust it for covered or uncovered 

 objects (Fig. 29). The front lens is fixed in the case a 

 the inner lenses, in b, e, and /, are 

 two lines engraved on the outer 

 case a, and d a line on a portion 

 of the inner case projecting through 

 a slit in a. For a covered object 



the lenses are approximated by a 

 screw until the line e is opposite d, 

 while, for an uncovered object, 

 they are separated until the line 

 e is opposite d. For ordinary FIG. 29. Objective with adjust- 

 lenses, however, this adjustment ^JfagF* and "" 

 is unnecessary, for one really 



scarcely ever looks at an uncovered object, and, moreover, 

 the adjustment is liable to shift and thereby throw things 

 into confusion. For very high powers, however, it is essen- 

 tial, because cover-glasses vary a good deal in thickness, 

 and if the cover be not of the very thickness with which the 

 optician corrected the lens, definition is impaired. In 

 that case, therefore, it is important to have an adjusting 

 screw on the lens. 



39. Dry and Immersion Lenses. -With a dry lens, 

 the rays pass through air between it and the cover-glass. 



With an immersion lens, some 

 fluid, such as water or glycerine, 

 is interposed. Very high powers 

 are now almost always con- 

 structed on the immersion sys- 

 tem, for one of its great advan- 

 tages is the conservation of 

 light. Fig. 30 explains this ; 

 in A the oblique ray r is seen 

 passing from the cover-glass past 

 the lens. In B it passes from the 

 cover-glass into water, the refractive index of which being 



FIG. 30. A. Dry. B. Immersion 

 lens. 



