12 DR. WOLLASTON'S DESCRIPTION OF A MICROSCOPIC DOUBLET. 



brought into the same plane as the object to be examined. This may perhaps 

 be most conveniently done by two tubes screwing one into the other. 



A stage for carrying the object, furnished with the requisite 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 necessary that the axes 

 of the lenses and the centre of the perforation 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 bull's-eye lanthorn may be used 

 with great advantage. 



With this microscopic doublet I have seen the finest striae and seri-atures 

 upon the scales of the Lepisma and Podui-a, and the scales upon a gnat's wing, 

 Avith a degree of delicate perspicuity which I have in vain sought in any other 

 microscope 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, having 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 im- 

 ])aired, but even impi'oved by the contact of the two media ; but if a double 

 convex lens be used, and it should accidentally touch the fluid, which not un- 

 frequently happens when the lens is of short focus, there is an end of the ex- 

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



London, 

 October 28th, 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 ap- 

 peared to me to be convenient, and which is represented at Plate II. fig. 3. A 

 tube of sufficient length and diameter forms the body of the instrument ; one 



