ON students' microscopes. 57 



cate, but a person accustomed to a rack movement fintls the 

 other awkward, and is apt to do harm with it. The tube 

 movement requires, of course, less workmanship, and therefore 

 diminishes the price, or ought to do so. 



4. Fine adjustment. — This is always effected by a screw 

 with a very fine thread, usually acting against a spring. It 

 is sometimes made to move the body of the instrument, 

 sometimes the tube, and sometimes merely the piece con- 

 taining the objective. The latter arrangement, acting by 

 means of a lever, is probably the best, and is found on most 

 English microscopes. 



5. Stage. — This part of the instrument varies exceedingly, 

 and is susceptible of almost any degree of complexity. If 

 the microscope is used vertically little or no mechanism is 

 necessary, but in the oblique position some arrangement for 

 holding the slide and permitting restrained motion becomes 

 indispensable. We do not pretend to decide what is the 

 best means of attaining these ends, but for the comfort of 

 those to whom simplicity is a matter of necessity we may ex- 

 press an individual opinion that an elaborate mechanical stage, 

 besides being by no means necessary, is far from being an un- 

 mixed advantage. The tube under the stage, for receiving 

 additional apparatus, which is almost peculiar to English 

 microscopes, is decidedly useful. Any arrangement for se- 

 curing the slide above ought to be removable at will. 



6. Mirror. — The additional expense of giving a double 

 mirror, one plane the other concave, is so very slight that no 

 student's microscope ought, we think, to be witliout this 

 convenience. The mirror ought, if possible, to have lateral 

 movement, as well as universal movement in the axis of the 

 microscope, for the purpose of getting good oblique illumina- 

 tion. 



7. Eye-pieces. — Of these little need be said, since the 

 amount of magnifying power desirable in them depends upon 

 what the objectives will bear. But a very high eye-piece may- 

 be occasionally most useful, though not convenient to work 

 with as a general rule. 



8. Objectives. — These are, it need hardly be said, the most 

 important parts of the whole instrument, and with thoroughly 

 good objectives the character of other parts is only of sub- 

 ordinate importance, while nothing can make up for inferio- 

 rity in this respect. Moreover, this is just the part in which 

 it is worth while to spend money, since it is not lost either 

 in mere display or in mere saving of trouble. The principal 

 questions, however, which the student has to consider are 

 what power is necessary for his purpose, and what objectives. 



