THE MICROSCOPE. 12T 



the highest optical attainment at the present time, are de- 

 signated as apochromatic. The lenses, entering into the 

 composition of these objectives, are made of the special 

 glasses mentioned, and in addition, there is present a lens 

 made of fluorite. 



The employment of a cover-glass, between the object 

 and the objective, disturbs the correction of the latter. 

 The rays of light, passing from the object through the 

 cover-glass, are refracted, so that the peripheral rays 

 which enter the objective, seem to come from a point 

 nearer the objective than do the central rays. This condi- 

 tion is overcome by making an under-corrected objective, 

 which will focus both these points at once. In objectives 

 provided with a "collar"' under-correction is obtained by 

 turning the collar so as to bring the back lenses nearer to 

 the front lens. The correction collar is placed only on the 

 dry, high power objectives. In the absence of a collar, the 

 necessary correction for a thick cover-glass can be obtained 

 by shortening the tube-length, thus causing the eye-piece 

 to approach the objective. Objectives are usually made 

 without a collar, and are corrected for a cover-glass having 

 a thickness of 0.17 mm. Markings on an object, which are 

 easily seen when a cover-glass of. this thickness is em- 

 ployed, disappear when an appreciably thicker, or thinner 

 cover-glass is used. Unlike the dry lenses, the oil immer- 

 sion objectives are largely independent of the thickness of 

 the cover-glass. 



It is evident that the objective is the most important 

 part of the microscope. Consequently, in purchasing a 

 microscope, special attention should be given to the quality 

 of the objectives. A good microscope means, above all, 

 good objectives. It is a common error for the student, to 

 believe that the best microscope is the one that magnifies 

 the most. This is far from being true. It is not tlie power 

 of magnification, but the ability to show up objects in their 



