Dr WoWaiitoii's Microscopical Doublet. 325' 



and the distance of the convex lens from the perforation may 

 be somewhat varied. The length here given, six inches, being 

 that which it was thought would be most convenient for the 

 height of the eye above the table. The diameter of the image 

 of the perforation A, need not, excepting with lower powers 

 than are here meant tpb^ considered* exceed one-twentieth of 

 an inch. 



The intensity of illumination will depend upon the diame- 

 ter of the illuminating lens, and the proportion of the image 

 to the perforation, and may be regulated according to the wish 

 of the observer. 



The compound magnifier M, consists, as before-mentioned, 

 of two plano-convex lenses ; the proportion of the foci of these 

 lenses being about as 3 to 1. They are fixed in their cells, 

 having their plane sides next to the object to be viewed, their 

 plane surfaces being distant from each other about 1/y or IJ 

 of the length of the shorter focus. This distance should be 

 varied by trial, until the utmost possible degree of distinctness 

 has been attained, not only in the centre, but throughout the 

 whole field of view. 



In ordpr to determine the distance between the plane sur- 

 faces of the lenses, I have used the following contrivance. A 

 wire is bent so as to form a spring, to the ends of which two 

 small pieces of plate glass are attached. Between the surfaces 

 of the pieces of glass is placed the interior cell, or that which 

 carries the lens of the longer focus ; and the distance between 

 the exterior surfaces of the pieces of glass is to be measured 

 with a pair of callipers : the cell is then to be screwed into its 

 place, and the compound cell subjected to the same operation ; 

 when the increase of distance between the exterior surfaces of 

 the pieces of glass will evidently be equal to the distance be- 

 tweeh the plane surfaces of the lenses. 



The exterior cell of the compound magnifier should be 

 formed with a flanch, so that it may rest upon the piece that 

 receives it. This is a far more convenient method than screwr 

 ing, and the magnifiers can be more readily changed. 



The lens E T, or the perforation A, should have an adjust- 

 ment by which th^ distance between them may be varied, and 

 the image of the perfora);ion be thus brought into the same 



