SSW» Dr Wollaston's Microscopical Doublet. 



plane as the object to be examined. This may perhaps be 

 most conveniently done by two tubes screwing one into the 

 otlier. 



A stage for carrying the object, furnished with the requi- 

 site means for lateral adjustments, is fixed at a, between the 

 magnifier and the lens E, T. The adjustment for distinct 

 vision is applied to the piece carrying the compound magnifier. 



For the perfect performance of this microscope, it is neces- 

 sary that the axes of the lenses and the centre of the perfora_ 

 tion A, should be in the same right line. This may be known 

 by the image of the perforation being illuminated throughout 

 its whole extent, and having its whole circumference equally 

 well defined. For illumination at night, a common buUVeye 

 lanthorn may be used with great advantage. 



With this microscopic doublet I have seen the finest striae 

 and serratures upon the scales of the Lepisma and Podura, 

 *and the scales upon a gnat's wing, with a degree of delicate 

 perspicuity which I have in vain sought in any other micro- 

 scope with which I am acquainted. 



Before I conclude^ I would point out one great advantage 

 that has confirmed me in the preference I have given to the 

 use of a plano-convex lens, properly employed ; that is, hav- 

 ing its plane side next to the object : namely, that if such a 

 lens should touch a fluid under examination, the view is not 

 only not impaired, but even improved by the contact of the 

 two media ; but if a double convex lens be used, and it should 

 accidentally touch the fluid, which not unfrequently happens 

 when the lens is of short focus, there is an end of the exami- 

 nation, until the lens has been taken out, wiped, and replaced. 



London, October 28, 1828. 



Appendix. 



The instrument which has been described will of course 

 admit of many varieties of form ; I shall, however, add a 

 description of that which has appeared to me to be convenient, 

 and which is represented in Plate III. Fig. 7. A tube of suf- 

 ficient length and diameter forms the body of the instrument ; 

 one end of the tube is closed by a piece having a screw S, by 

 means of which it may be fixed in the top oi' the box intend- 



