THE MICROSCOPE. 29 



ground to the same radius, and applied by their plane sur- 

 faces to a flat piece of metal, having an aperture of the same 

 diameter as would be suitable for a lens of equal size, 

 but composed of one piece of glass; and the size of the 

 aperture which, on experiment, was found always to give 

 the best definition, was about one-fifth part of the focal 

 length in diameter. This form of doublet was subsequently 

 improved on by Sir David Brewster, who, instead of using 

 the flat piece of metal, and two planoconvex lenses, employed 

 two hemispherical lenses, cemented to the ends of a tube of 

 brass, and filled all the interspace with a fluid of the same 

 refractive power as the glass. This led Sir David to the idea 

 of the grooved sphere, which is nothing more than a spherical 

 lens having a deep groove cut round it in a plane per- 

 pendicular to the axis of vision; a plan analogous to that 

 of the Coddington lens. Experiments on doublets were 

 now carried on by Sir John Herschell, Sir David Brewster, 

 Mr. Coddington, and others, and we have various forms 

 recommended for use by each of these gentlemen; by the 

 former we have three, viz., the periscopic doublet, consisting 

 of a double convex lens of the best form, but placed in 

 its worst position (radii as 6 to 1) for the lens next the eye, 

 and a planoconcave, whose focal length is to that of the 

 other, as 2, 6 to 1, or as 13 to 5, placed in contact with its 

 flatter surface, and having its concavity towards the object. 

 The second consisted of the planoconvex doublet, which is 

 made with two convex lenses of equal focal lengths, the 

 convex sides being placed in contact, and the eye and object 

 opposite the plane sides; and the third, the doublet of no 

 aberration, consisting of a planoconvex lens, and a meniscus 

 placed in such a manner, that the convex sides of both 

 were in contact. This latter form of doublet Sir John pro- 

 poses as the best for obtaining perfect distinctness in micro- 

 scopical observations, and Mr. Pritchard states:*— « That 

 doublets of this kind answer remarkably well, but their angle 

 of aperture is small as compared with combinations of double 

 achromatics." 



* Microscopic Cabinet, p. 163. 



