162 MICROSCOPIC ILLUSTRATIONS. 



At the same time I willingly admit, that if the body is 

 made very long or large it will be almost impossible to 

 combine the stability requisite for practical purposes 

 with a capability of traversing and adjustment. 



It has been asserted, that if after we have adjusted our 

 light for viewing a transparent object satisfactorily, we 

 throw the axis of the optical part out of the position 

 which we first selected, we must begin our work of illu- 

 mination over again, or lose the effect of the original 

 one. This is true beyond certain limits, but the body of a 

 microscope may be moved very considerably, without in 

 the least affecting the vision of ordinary transparent 

 objects, and to opaque bodies, of course, the observation 

 cannot apply. Further, it is said that the employment 

 of cones and diaphragms below the stage is of vast use 

 in developing the nature of many transparent bodies, 

 and in tempering the light to the shade to produce a 

 maximum of distinctness ; and that if these are employed, 

 the most perverse and prejudiced antagonist cannot re- 

 fuse to admit that any diversion of the optical part from 

 the line of adjustment must utterly destroy, not only 

 distinctness, but nearly all vision whatever. 



To obviate these objections let there be a large roomy 

 slider-holder, with a good steel spring, so as to pinch the 

 plates together pretty tight ; the object can then be moved 

 about by the hands in any direction, and the body may re- 

 main a fixture, save as to the adjustment of the focus. 

 I have always found that people like to move an object 

 about with their hands, even when they have screw mo- 



