588 Notes. 



and lasted many years, for it is found in the Lister-Tulley,* the 

 first achromatic Microscope, and again in the Eoss-Valentine,f and 

 we still find it in the Ross dissecting Microscope \ of 1855. 



We now pass on to the triangular ^rismatic limb. The first 

 Microscope to have a triangular liw.j was the large Benjamin 

 Martin Microscope, in our Cabinet, the date of which may be 

 placed at 1770. This limb was fixed, and the stage, for the instru- 

 ment was a stage focusser, racked up and down upon it. The next 

 time we hear of a triangular limb is in Varley's description § of 

 a " Microscope for live Objects," this instrument was made by 

 Powell. Although, for reasons stated, Varley did not apply the 

 triangular limb to his Microscope, he says, " my late uncle about 

 thirty years ago introduced the triangular bar and triangular 

 tube. ..." 



In Valentine's Microscope, made by Ross (1831), the tri- 

 angular limb, however, is reintroduced. Again we have it in 

 Pritchard's Microscope, figured in his Microscopic Cabinet 1832 ; 

 this Microscope was made by Powell, and was a modification of 

 Varley's. So we see that Varley suggested the reintroduction of 

 the triangular limb, Ross first adopted it in Valentine's Microscope, 

 and Powell, following Ross' lead, used it in the Microscopes he 

 made for Pritchard. 



There is no coarse adjustment, but a fine adjustment screw, 

 placed at the bottom of the limb, acts directly on the triangular 

 focussing bar. This part is copied from Varley's Microscope, but, 

 as his sprung nut || is omitted, the loss of time is very great. 



Beneath the stage is a sub-stage condenser in a sliding tube 

 fitting ; its optical part consists of a sliding convex lens. 



The objective is a single bi-convex lens of 1 in. focus ; it is 

 mounted precisely like the Wollaston doublets of that period. The 

 gauge of the mount is 0*618 in., and some similar, but signed, 

 examples of Andrew Pritchard were found to vary between * 614 

 and 0*619. The foot is circular; we find that Microscopes on 

 circular feet are figured in the second edition of Pritchard's Micro- 

 scopic Illustrations, 1838, pp. 82 and 88, figs. 11 and 12. 



I have examined the tongue of a blow-fly with this instrument, 

 and was quite surprised at the high quality of the image. 



This Microscope was probably made by Powell for Andrew 

 Pritchard, circa 1835-40. 



This instrument possesses two points of interest. 



1. It is an undoubted early example of the reintroduction of 

 the triangular focussing bar. 



2. It is also an early example of a circular foot. 



* Journ. R.M.S., 1900, p. 551, fig. 146. f Op. cit., p. 425, fig. 104. 



X Idem, p. 428, fig. 109, and Quart. Journ. Mic. Sci., toI. 3, p. 220, fig. 15. 

 § Trans. Soc. of Arts, vol. 48, p. 12 (1832), aud Journ. K.M.S., 1900, p. 283, 

 figs. 70, 71. || Journ. E.M.S., 1900, P . 284, fig. 72. 



