282 Transactions of the Society. 



NOTES. 



The Microscopes of Powell, Ross, and Smith. 



By Edward M. Nelson. 



i. — hugh powell's microscopes. 



Hugh Powell, born in 1799, was, like Andrew Ross, a philosophical 

 instrument maker. He resided at 24 Clarendon Street, Somers Town, 

 and probably worked for the trade, but the first notice we have of him 

 is in connection with the first of three Microscopes made by him for 

 ■Cornelius Varley.* These Microscopes are figured and described in the 

 Transactions of the Society of Arts, the account of the first being in 

 vol. xlviii. p. 332, pi. 4 (1831). 



Figs. 70, 71 show that this is a non- achromatic simple Microscope ; 

 five lenses are mounted in a calotte rotating wheel, the coarse adjust- 

 ment is by moving the stage up and down the pillar, and by clamping 

 it with a pinching screw. The rod carrying the lenses, which slides 

 into the top of the pillar, is moved by a line adjustment screw, the 

 milled head of which is seen at the bottom of the pillar. 



The plan of this fine adjustment is very important, because it is 

 the first instance we have of a sprung fine adjustment movement ; a 

 nut is placed on the screw at a short distance from its entry into the 

 lower end of the rod which carries the lenses (fig. 72), and by it a 

 spring is compressed (tig. 72) ; another spring is also placed at the 

 lower end of the screw to keep the milled head in close contact with 

 its seat (fig. 73). JBy these means all slack between the movable rod 

 and the screw is taken up, thus ensuring that a movement of the rod 

 will immediately follow that of the screw, and this quite independently 

 of the direction in which it is turned ; in other words, loss of time 

 when the motion of the screw is reversed is wholly prevented. We 

 shall see that this device was copied in Valentine's Microscope, made 

 by Andrew Ross. The stage has a lever mechanical movement ; 

 this, after having been twice modified by Varley, was further altered 

 by White in 1843, and in that iorm was used by several makers for 

 about thirty years. Below the stage there was a cylinder diaphragm 

 (fig. 74). This is the first time we meet with a true cylinder dia- 

 phragm ; the appliance which most nearly approaches it being the 

 " cannon " of Joblot in 1719. The base of this Microscope is peculiar, 

 as it has no proper foot, but instead it is fitted with a screw-clamp for 



* Cornelius Varley was an artist; he was born in 1781 and died in 1873, his last 

 picture having been paiuled in his ninetieth ytar. He was much interested in scien- 

 tific matters ; he ground and polished Microscope lenses, and in 1826 he ground and 

 polished a plano-convex diamond lens for Dr. Goring. He was one of the seventeen 

 founders ot this Society who met at Edwin Quekett's house in 1839. 



