30 



PEACTICAL TREATISE ON 



By far the most important contribution to microscopical 

 science at this period, was the microscopic doublet, the in- 

 vention of Dr. Wollaston, it is described in the Philosophical 

 Transactions for 1829,* and the mode of illumination therein 

 recommended, gave to the single microscope an importance and 

 degree of usefulness, which it had never yet received in this 

 or any other coimtry. The doublet of Wollaston consisted of 

 two planoconvex lenses, having their focal lengths in the pro- 

 portion of 1 to 3, and placed at such a distance from each other, 

 as was ascertained to be best by experiment. It is said that 

 he was led to this invention by a knowledge of the construction 

 of the achromatic Huyghenian eye-piece, which, if reversed, 

 would make a microscope ; but impaired health caused hiTn to 

 communicate his paper to the Boyal Society earlier than he 

 at first intended, and his premature death deprived him of the 

 satisfaction of ever witnessing the great improvement subse- 

 quently made in his doublet, by the introduction of a stop or 

 diaphragm between the two lenses. The microscope stand, 

 with which the doublet was used, was as simple and as elegant 

 in its construction as the doublet itself; and is 

 shown in section, by figtire 19, where A B re- 

 presents a brass tube, about six inches long and 

 an inch or more in diameter, capable of being 

 screwed into the cover of a box or stand, by the 

 screw D. At C a circular perforation is made for 

 the purpose of admitting the light to the mirror 

 E. Above the mirror at F is a diaphragm or 

 stop, for cutting off the outer rays of light 

 reflected from the mirror. At the upper end 

 of the tube is a planoconvex lens of about three 

 quarters of an inch focal length, set in a metal 

 frame at G, with its plane side uppermost; 

 its use being to bring the rays of light to 

 a focus on an object placed across the top of 

 tube at P, which acts as a stage. At I is 

 fixed a small rack, upon which an arm, H, 

 carrying the doublet, M N O, can be moved up 

 * Philosophical Transactions, 1829, p. 9. 



Fig. 19. 



