196 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



The present Editor has had one of these microscopes in constant, and 

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

 most delicate work can be 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 very best clip that can be used the pivots of the 

 mirror, and the carefully sprung 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 a finished 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 

 Huyghenian 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. 



2. The mechanical tube- 

 length. 



FIG. 158.-Powell and Lealand's sub-stage In order that the binocular 

 with fine adjustment (1882). may suit persons with ' narrow 



centres' to their eyes, the dis- 

 tance between them should not be greater than 2^ inches. The 

 mechanical tube -length is 8| inches for the standard tube. When 

 the eye-pieces were ' home ' in their places in the tubes they j ust 

 touched each other, the inner sides of the binocular tubes being cut 

 away ; so under the above conditions a larger field than is thus 

 obtained is simply impossible. The size of the field-glass deter- 

 mines the size of the eye-piece, and that was made to fix the 

 diameter of the body-tube. 



Very wisely these makers made the tube of the sub-stage the 

 same size, so as to have one gauge of tubing throughout. This 

 allows a Kellner or other eye-piece to be used as a condenser, thus 

 reducing the number of adapters. 



Lately this firm have altered their sub-stage tube to a gauge 

 recommended by the Royal Microscopical Society. This involves 

 an adapter where the sub-stage apparatus was adapted to the old 

 gauge, or when an eye- piece is used as a condenser ; as the size is 

 too large for a binocular. 



The Ross model, in its completest form as left by Andrew Ross, 



