50 SELECTION AND tJSE 



The Jackson Model. The special characteristic of this 

 model lies in the fact that the " ways " of both the coarse and 

 fine movements are brought as close as possible to the optic axis 

 of the instrument. In the Ross model the coarse movement is at 

 a considerable distance from this axis, and any lateral motion 

 which may take place on the sliding part is greatly magnified 

 at the axis of the body the tram verse bar, which supports the 

 latter, acting like the long arm /)f a lever to greatly multiply 

 that which at first was very insignificant. Hence the advan- 

 tages which the Jackson model presents are great steadiness and 

 the fact that, in common with the Ross model, it affords abun- 

 dant room underneath the stage for those accessory methods of 

 illumination which are indispensable in the highest class of 

 work. It is almost impossible to attach to the smaller patterns 

 of the Continental model a convenient sub-stage, carrying 

 polarizer, achromatic condenser, paraboloid, etc., while the 

 model under consideration is specially designed to receive 

 these important accessories. 



This model is a very general favorite both with English 

 and American makers. Amongst the latter it has been adopted 

 as the best for all first-class stands. The engraving, Plate II, 

 shows a very excellent form of this model, made by James W. 

 Queen & Co. It will be seen that along a great part of its 

 length the body is attached to the rack-bar, and this bar slides 

 on long ways, which give great steadiness and freedom from 

 vibration. A special feature of this stand is the low position 

 of the milled head for the fine adjustment. This enables the 

 microscopist to rest the hand on the table so as to avoid much 

 of that slight but imperceptible vibration which the unsup- 

 ported hand is apt to impart. 



Tlie Continental Form. Most of the stands made by 

 the better class of French, German, and Austrian microscope 

 makers are characterized by a low, compact form, and great 

 solidity and simplicity of construction. They are intended to 

 be used chiefly in a vertical position, and hence the bodies 

 are short, and the space beneath the stage is contracted to the 

 last degree. And as a low price is an important feature in these 

 microscopes, the coarse movement is generally secured by 



