28 Modern Microscopy. 



THE LIMB. 



The limb of the instrument is another important detail. 

 It will be noted by the illustration (Fig. 5) that there is 

 no support for the body-tube from the bar to the top of the 

 eyepiece, and unless this bar be most perfectly fitted, and 

 every detail of construction considered, with such an amount 

 of leverage as there is at the eyepiece end a very apparent 

 shake will soon be noticed. The limb fitted to the other 

 instruments shown, known as the Jackson form, is there- 

 fore recommended, as it supports the body from one end to 

 the other, and at the same time is as firmly connected with 

 the other working parts of the instrument as the bar form. 



THE BODY-TUBE. 



As to the size of the body. It has always been the 

 custom, in the construction of English microscopes, to make 

 the body of fairly large size ; while on the Continent the 

 reverse is the case, and it is made as small as possible. 

 Owing to the introduction of Continental oculars and objec- 

 tives, English makers have in recent years adopted the 

 Continental diameter of body to a considerable extent. For 

 photographic purposes it is held by some workers that a 

 fairly large body should be used, but for ordinary visual 

 work the writer has never experienced any advantage from 

 the large over the small tube. It may here be said that 

 Continental instruments have their bodies constructed very 

 much shorter than the English forms, the rule being to 

 adjust their objectives to a tube 160 millimetres long, while 

 English opticians adjust theirs to one of 250 millimetres. 

 The microscopist who enters enthusiastically into his work 

 invariably has objectives of both Continental and English 

 make, and he therefore requires the convenience of being 

 able to use both perfectly. One or two English opticians 

 make microscopes with a body of 160 millimetres, and a 



