58 THE MICROSCOPE. 



drawn out increases the magnification of the image, 

 without having to change the eye-piece. When using 

 the micrometer eye-piece, we are enabled by the aid of 

 the draw-tube to fill the whole field of view and make 

 a precise comparison between the divisions of the eye- 

 piece and the stage micrometer. In Messrs. Beck's 

 microscopes, the draw-tube is furnished with a rack- 

 and- pinion movement for the purpose of facilitating 

 adjustment. 



The Object-glass. The microscope depends so much 

 for its effectiveness and general utility upon the perfec- 

 tion of the object-glass or objective, that it is absolutely 

 necessary for any one about to use the instrument to 

 make himself perfectly familiar with its relative quali- 

 ties. It will scarcely be possible to form any just 

 estimate of the value of this or that maker's objective 

 by a comparison of magnification ; indeed the propor- 

 tional amplification of the object-glasses of the most 

 conscientious opticians will on comparison be found to 

 differ materially. 



To arrive at a literally correct judgment of the value 

 of an objective to the microscopist, there are special 

 qualifications by which it should be judged. These, 

 for the sake of convenience, may be divided into : 

 Its defining power; its penetrating power, or focal 

 depth ; its resolving power ; its working distance and 

 its flatness of field : all of which are qualifications of 

 the greatest importance, especially when an objective 

 is about to be employed in scientific researche. 



The Defining Power of the objective depends upon 

 the perfection with which the corrections of its chro- 

 matic and spherical aberrations are made. When these 

 are nicely balanced, the image will be sharp, and the 

 minutest details of an object seen with greater clear- 

 ness. Whatever other qualities may be absent in the 

 object-glass, fine definition must be secured. This 

 quality may be tested by taking a known test-object, 

 as it is termed, a blood corpuscle, a diatom mounted 

 in balsam, or a Podura-scale, and comparing the sharp- 

 ness and perfection of the image produced by one 

 objective, against that of another. 



Penetrating Power, or focal depth, is a quality 



