MECHANICAL REQUIREMENTS. 43 



every variety of focal distance, xoithout movement of the object. 

 It is a principle now universally recognized in the construction of 

 good Microscopes, that the Stage whereon the object is placed 

 should be a fixture ; the movement by which the Focus is to be 

 adjusted being given to the Optical portion. This movement should 

 be such as to allow free range from a minute fraction of an inch to 

 three or four inches, with equal power of obtaining a delicate ad- 

 justment at any part. It should also be so accurate, that the 

 optic axis of the instrument should not be in the least altered by any 

 movement in a vertical direction ; so that if an object be brought 

 into the centre of the field with a low power, and a higher power 

 be then substituted, it should be found in the centre of its field, 

 notwithstanding the great alteration in the focus. In this way 

 much time may often be saved by employing a low power as a 

 finder for an object to be examined by a higher one ; and when an 

 object is being viewed by a succession of powers, little or no 

 readjustment of its place on the stage should be required. For 

 the Simple Microscope, in which it is seldom advantageous to use 

 lenses of shorter focus than l-4th inch (save where Doublets are 

 employed, § 18), a racJc-and-pinion adjustment, if it be made to 

 work both tightly and smoothly, answers sufficiently well ; and 

 this is quite adequate also for the focal adjustment of the Com- 

 pound body, when Objectives of low power only are employed. But 

 for any lenses whose focus is less than half an inch, a 'fine adjust- 

 ment,' or ' slow motion,' by means of a screw-movement operating 

 either on the object-glass alone or on the entire body, is of great 

 value ; and for the highest powers it is quite indispensable. In 

 some Microscopes, indeed, which are provided with a ' fine adjust- 

 ment,' the rack-and-pinion movement is dispensed with, the 'coarse 

 adjustment' being given by merely sliding the body up and down 

 in the socket which grasps it ; but this plan is only admissible where, 

 for the sake of extreme cheapness or portability, the instrument 

 has to be reduced to the form of utmost simplicity (Figs. 40, 41). 

 Where only one means of focal adjustment is provided, this is 

 best afforded by the substitution of the chain-movement for the 

 rack-and-pinion, as in Mr. Ladd's Microscope (Fig. 36). This has 

 the advantage of being smoother and more sensitive, of being less 

 likely to become unequal by wear, and of being easily tightened if 

 it should ' lose time ; ' whilst its delicacy and smoothness admit of 

 an exact adjustment being made by its means alone, even when 

 moderately high powers are employed, in the manner to be 

 presently described. It will be shown hereafter that the use of 

 the ' slow motion ' is by no means restricted to the exact adjust- 

 ment of the focus ; and it cannot be advantageously dispensed 

 with in a Microscope which is to be used for any but the most 

 common purposes. 



in. Scarcely less important than the preceding requisite, in the 

 case of the Compound Microscope, though it does not add much to 



