Obituar/f. 175 



which is of the ordinary Continental direct-acting screw type, has 

 the action of its compensating spring reversed, i.e. it acts down- 

 wards instead of upwards ; but, what is more important, a feature, 

 now quite common, is seen in the substage, which is of the swing- 

 out, screw-focusing, and centring type. This requires an explana- 

 tion. At first the Abbe condenser was fitted by Zeiss merely with 

 a push-focusing slide, without centring gear ; but there was in ad- 

 dition a cylinder diaphragm, fitted both with rackwork focusing and 

 centring gear on a swing-out arm. This plan was reversed by 

 Eeichert,* who placed the cylinder diaphragm in a slide and the 

 condenser in the swing-out arm with a spiral-focusing screw ; he 

 however suppressed the centring gear, and centred the movement 

 in the nominal axis by a pin. M. Nachet followed Eeichert's 

 improvement, mounting his condenser in a spiral-screw focusing 

 swing-out arm, but by retaining the centring gear he has the 

 priority for this very useful appliance for students' cheap Micro- 

 scopes. 



Notwithstanding the interest he took in all that concerned the 

 Microscope, he found time to pay attention to other branches of 

 optics, especially that connected with ophthalmology. All this 

 time his scientific knowledge and his great practical skill were 

 always at the service of those who required aid for any special 

 and difficult work : indeed it was owing to this energy and com- 

 plaisance that he participated in so many and so various re- 

 searches. 



In 1899 he retired from business, and this retirement allowed 

 him leisure to prosecute his scientific researches, to gather to- 

 gether numerous notes concerning the past history of the Micro- 

 scope, notes which he had collected little by little, relating to 

 interesting models ; and also to form a library of rare books on the 

 same subject. 



Brought up among savants, a contemporary of others in whose 

 works he had shared, having assisted in the scientific evolution of 

 nineteenth century, it may easily be imagined that the remarkable 

 memory with which he was endowed was an inexhaustible mine of 

 interesting remembrances. Alfred Nachet became a Fellow in 

 1879, and in 1906 presented to the Society one of its most valu- 

 able and interesting possessions, namely, six Micro-Daguerreotypes 

 taken with the Electric Light by Leon Foucault in 1844. The 

 letter which accompanied the donation is dated January 14, 1906, 

 and is recorded in the Proceedings, pp. 122-3, of the Society's 

 Journal for 1906. 



* See this Journal, ser. 2, vol. iv. (1884) p. 438, fig. 53. The swing-out arm was 

 suggested by the swing-out arm which carries the polariser in a petrological 

 Microscope. 



