DR. WOLLASTON'S DESCRIPTION OF A MICROSCOPIC DOUBLET. 11 



mirror R, and which is to be brought to a focus at a, giving a neat image of 

 the perforation A at the distance of about eight tenths of an inch from the lens 

 E, T, and in the same plane as the object which is to be examined. The length 

 of the tube 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 to be considered, exceed one-twentieth 

 of an inch. 



The intensity of illumination will depend upon the diameter of the illumi- 

 nating 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 1t% or U 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 order to determine the distance between the plane surfaces of the lenses, 

 I have used the following contrivance. A wire (Plate II. fig. 2.) is bent so as 

 to form a spring, to the ends of which two small pieces of plane glass are 

 attached. Between the surfaces of the pieces of glass is placed, in the manner 

 represented in the plate, 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 between 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 conve- 

 nient method than screwing, and the magnifiers can be more readily changed. 



The lens E,T, or the perforation A, should have an adjustment by which the 

 distance between them may be varied, and the image of the perforation be thus 



c2 



