EYE PIECES: OBJECTIVES 



111 



iiiicus, which are to be differentiated by 

 means of color rather than by outline. 

 For the best work it is an essential ad- 

 JTUift to the microscope. (Fig, 4.) 



Fig. 5. Eye Pieces. 



In choosing a microscope for use in the 

 Inborator}^ a few cardinal principles may 

 be borne in mind. As many differences 

 of opinion will be found among working 

 microscopists, the question must be left 

 to optical principles; for any microsco- 

 pist, after working many years with a 

 particular type of instrument, may be- 

 come so proficient in its use that, not- 

 withstanding rndical errors in its con- 

 struction, good work can be done. Indi- 

 vidual bias should not, therefore, govern 

 the purchaser. 



Firmness and solidity are virtues in a 



microscope, and as these qualities are 

 given by the base, the pillar, and the 

 arm, these parts should be strong and 



solid, not necessarily massive. We pre- 

 fer the so-called Continental type, as 

 giving compactness, firmness and solid- 

 ity. 



The stage should be ample; a square 

 is perhaps preferable, and it should be 

 thick and firm. The investigator will 

 often use corrosive substances in his 

 work, and the stage would better be of 

 metal. 



A coarse adjustment, preferably a rack 

 and pinion, that will not wobble, and 

 that will not allow the tube to descend 

 of its own weight, is a requisite. A fine 

 adjustment is an essential; it should 

 work evenly and smoothly, and is prefer- 

 ably placed at the top of the pillar. 



A nose piece, double or triple, is now 

 considered a necessity. Care should be 

 taken that it centres the objectives and 

 that its joints do not leak. 



As the objective is the most important 

 part, a few words regarding its choice 

 may be of benefit. Two errors of con- 

 struction should be borne in mind; these 

 are chromatic and spherical aberration. 



Chromatic aberration is a defect due 

 to the double action of a lens, by which 

 it acts as a magnifier and as a prism. 

 Acting as a prism, it decomposes the 



Fiff. 6. Construction of Objective. 



light into its elements, and if a lens 

 shows bauds of color around the edgea 

 of the object looked at it has not been 

 totally "corrected" for chromatic aber- 

 ration. 



