nAwuwV 



2S6 Transactions of the Society. 



the descriptions of these Microscopes will show that Powell's share in 

 this matter was far from inconsiderable. 



Next we have, in the Transactions of the Society of Arts, the first 

 description of Powell's work issued under his own name, viz. his stage 

 fine adjustment.* Here there is a Turrell stage raised by wedges, 

 which are advanced beneath it by means of a micrometer screw. 



The head of the screw is divided into twenty division?, and the 

 value of each division is one six-thousandth of an inch. The Micro- 

 scope made by Powell for this Society in 1841 has a stage of this kind 

 (fig. 76). 



The next in order is Varley's second Microscope, which he calls 

 a " Vial Microscope."! This Microscope, simple and non-achro- 

 matic, was a modification of his previous model ; both the calotte 



lens-holder and the fine adjust- 

 FlG - ll - ment were suppressed, and a 



rackwork coarse adjustment with 

 a square bar having 40 teeth to 



the inch and a sprung pinion was 



supplied ; this is the first instance 

 we have of a sprung pinion, and a better plan of mounting a pinion 

 has not yet been devised ; but although it was subsequently adopted 

 by the principal makers, all, with the exception of Powell, have at 

 the present time discarded it (fig. 77). (A sprung pinion can also 

 be very well seen in fig. 80.) 



There was a special kind of stage for holding a cylindrical bottle 

 or phial, through the side of which algae or other objects were 

 examined ; for viewing slides or similar objects, the phial was removed, 

 and a stage with lever mechanical motion fitted into its place. The 

 screw-clamp foot was replaced by a circular ring and post, not unlike 

 a modern Microscope lamp support. In this Microscope we first have 

 the mirror-arm joints sprung in the manner indicated above (fig. 75). 

 These Microscopes became popular, and in a modified form were made 

 by Powell for Andrew Pritchard, who published an account of them 

 in the second edition of his Microscopic Illustrations (1838). 



Now we come to the year 1841, which is an important date in 

 Microscope construction, because about this time the recently con- 

 stituted Microscopical Society of London " requested Messrs. Hugh 

 Powell, Andrew Koss, and James Smith % each to furnish a standard 

 instrument, made according to their own peculiar views." Powell's 

 Microscope was delivered on December 22nd of that year, and although 

 an account with a figure of it was promised, it has, so far as I am 

 aware, never been published. The Microscope (fig. 76) is still in our 

 cabinet, and, with the exception of the addition of a binocular body in 



* Vol. 1. part 2, p. 108, plate 3 being in vol. xlix. (1833). 

 t Trans. Soc. Arts, 1. p. 158, pis. 5 arid 6 (1834). 



j The order was given to Smith on August 19th, 1840, and to Powell and Ross 

 on May 26th, 1841. 



