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 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 
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- 
paired, 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 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, 
Octobei ’ 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 
