264 Dr. Goring's Commentary on 



Par. 9. ** As far as my trials have hitherto gone, I am led to consider tne 

 proportion of three to one as nearly the best for the relation of the foci of 

 these lenses, and their joint performance to he the most perfect, Vf\im the 

 distance between their plane surfaces is about 1 4-1 Oths of the shorter 

 focus. But as all the lenses I possess are not similar segments of spheres, 

 or of the same relative thickness, I could not expect exact uniformity in 

 the results." Again, par 12. " The compound magnifier, too, 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 sur- 

 faces being distant from each other about 1 4-1 Oths, or 1^, 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:" 



Microscopic doublets are of considerable antiquity ; there 

 are many better, as well as worse, than Dr. Wollaston's. 



Mr. B. Martin, in chapters 2 and 3, article 96 to 125, has 

 given a complete theory of distinct vision, by that combination 

 of glasses selected by Dr. Wollaston. His computations are 

 for the object-glasses of telescopes, hut these being reversed, or 

 used in the opposite direction, of course make microscopic 

 doublets ; because, diverging rays issuing from the focus of 

 the glasses will travel backwards by the same route in which 

 the parallel rays arrived at it; the result of Mr. Martin's 

 analysis is as follows : if two plano-convex lenses have their 

 convex surfaces opposed to parallel rays, and their foci equal 

 3, and 2, respectively^ the lens of the longer focus being 

 placed first, with an interval between them equal to 1, so that 

 their foci shall coincide on the same point, their spherical 

 aberration will be reduced to one quarter of that of an equi- 

 convex lens of the same focus and aperture. Moreover^ if 

 the lens 3 is supposed to be constant, and the lens 2 variable, 

 it matters not what its focus is, (if it does not exceed that of the 

 lens 3, provided always that it is so posited, that its focus 

 shall coincide with that of the other, which is its best pdsi- 

 tion.) The magnifying power and dis^nctness of the combi- 

 nation always remaining the same, an infinity of different 

 proportions may be selected, 'ztU equally' ^6od'J" it' 'fdUbiv^, 

 therefore, that two plano-convexes of the ^ame /ocw^ nijiy be 



