196 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



The present Editor lias had oneofthe.se microscopes in constant, and 

 often prolonged and continuous, use for over twenty years, and the 

 mo>t delicate work can he done with it to-day. It is nowhere 

 defective, and the instrument has only once been ' tightened up' in 

 some parts. Even in such small details as the springing of the 

 sliding clip the \ery best clip that can be used the pivots of the 

 mirror, and the carefully X/U-I/IHJ conditions of all cylinders intended 

 to receive apparatus, all are done with care and conscientiousness. 



An instrument of this kind may be made to appear perfect to 

 the eye, but at the same time may lack some most important elements 

 as .-( linished instrument. But this is an instrument of the highest 

 order as such, and at the same time a very fine specimen of highly 

 finished brass work. 



A note must be made before 

 leaving this microscope upon the 

 size of the tubes in the body and 

 the sub-stage. 



Powell and Lealand were the 

 only makers whose gauge of 

 tubing had a raison d'etre ; the 

 size of the tube was such that it 

 would take in a binocular body a 

 Iluvidieiiian 2 -inch eye-piece, 



having the largest field-glass pos- 

 sible. The size of this field-glass 

 depends on two factors. 



1. The distance between the 

 centres of the eyes. 



-. The mechanical tube- 

 length. 



In order that the binocular 

 may suit persons with ' narrow 

 centres' to their eyes, the dis- 

 tance between them should not be greater than :H 'inches. The 

 mechanical tube-length is s : i inches for the standard" tube. When 

 (lie eye-pieces were -home' in their places in the tubes they just 

 touched each other, the inner sides of t he binocular tubes being' cut 



FIG. 158. Powell and Lealand's sub stage 



with line .nl JM-.I incut i 1882). 



away; so under the a hove conditions 



a larger fieh 



obtained is simply impossible. The size of the 

 mines Ihe size of the eye piece, and that was made 



than 



diameter of t he h<>,l\ t ube. 



\\i-ely these makers made the tube of the 



tubing 



Used as 



field-glass 

 ' to 



is thus 

 deter- 

 fix the 



same size, so as to have one gau^e of 



sub-stage 



the 



This 

 a condenser, thus 



throughout. 



allows a Kellner Or other e\e piece to b, 



!< liH'ino- the number of adapters. 



l-alely this linn ha\e altered their snh-staye tube to a yauge 

 tnmended by the Royal M irmscopical Society. This involves 



lapter where the sub-stage apparatus was adapted to the old 

 wl "' n an eye piece is used as a condenser; as the size is 

 large for a binocular. 



'- iu its completes) form as left by Andrew Ross, 



