STEADINESS OF THE MICROSCOPE 157 



With this object before us we shall facilitate its attainment by 

 at once considering what are the essentials of a good microscope. 

 What are the attributes of the instrument without the possession of 

 which it cannot meet modern requirements ? 



T. Steadiness is absolutely indispensable : this would, in fact, 

 appear to be obvious. But we are bound to admit that it is, in what 

 sometimes claim to be stands of the first class, disregarded ; and when 

 the height of the centre of gravity in the English and American 

 stands of the first class is considered', this is a fatal mistake. 



It is pointed out in the section on micrometry J and drawing 

 that the optic axis of the microscope should be ten inches from the 

 table ; therefore a first-class microscope whose optic axis when 

 placed horizontally is either more or less than this is found wanting 

 in a material point. But to possess this characteristic it must have 

 n high centre of gravity. 



Now it is possible to secure steadiness by (1) weight or (2) 

 design. The Continental method has invariably been w r eight. The 

 pillar of the instrument is fixed to a cumbrous metal foot of horse- 

 shoe form, which bears so high a ratio to the whole remainder of 

 the instrument that it is usually steady. This secures the end 

 certainly, but by coarse and unwieldy means. It promises little 

 for the instrument as a whole. 



What is wanted is the maximum of steadiness w r ith the minimum 

 of weight. An old plan designed by Guff, circa 1765, of rotating the 

 foot below the pillar has been frequently reinvented. It was used 

 by Adams 1771, by Ross 1842, by Sidle and Poalk in America 1880, 

 by A. McLaren 1884, and recently again by Ross. This is a very 

 simple method of obtaining great stability for the instrument when 

 in either the vertical or horizontal positions. An instance of this form, 

 made by Andrew Ross in 1 842, is given in fig. 123 : the foot is seen to 

 be circular, with a vertical pillar attached eccentrically to it, and the 

 1 >ase rotates, securing stability in either a vertical or inclined position. 



Palpably, the mechanical compensation for the difficulty of an 

 elevated centre of gravity is an extended base. The leading fault 

 of many stands claiming the first rank is their narrowed bases. A 

 broad base, resting on three points only, and these plugged with 

 cork, is the ideal for a perfect instrument. 



II. Next in or dei- to the stand of the microscope comes what is 

 known as the body of the instrument the tube or tubes for receiv- 

 ing the objective at one end and the eye-pieces at the other. The 

 tube of the monocular is always provided with an inner tube called 

 the draw-tube. In a first-class instrument this latter should always 

 be provided with a rack-and-pinion motion, and should have a scale 

 of from two to three inches, divided into tenths or millimetres. This 

 enables the operator the more accurately to adjust apochromatic ob- 

 jectives so sensitive, for their best action, to accurate adjustment of 

 tube-length. In fact, it is always important to remember that ob- 

 jectives are corrected for a special tube-length ; that is to say, for 

 the formation of the image at a certain definite distance. 



1 Chapter IV. 



