PRINCIPLES AND THEORY OF MICROSCOPIC VISION 37 



the eye, and the lens of shortest focal length nearest the object. In 

 Dr. Wollastori's original combination no perforated diaphragm (or 

 ' stop ') was interposed, and the distance between the lenses was left 

 to be determined by experiment in each case. A great improvement 

 was subsequently made, however, by the introduction of a ' stop ' 

 between the lenses, and by the division of the power of the smaller 

 lens between two (especially when a very short focus is required), so 

 as to form a triplet, as first suggested by Mr. Holland. 1 When 

 combinations of this kind are well constructed, both the spherical 

 and the chromatic aberrations are so much reduced that the angle 

 of aperture may be considerably enlarged without much sacrifice of 

 distinctness ; and hence for all, save very low powers, such * doublets ' 

 and ' triplets ' are far superior to single lenses. These combinations 

 took the place of single lenses among microscopists (in this country 

 at least), who were prosecuting minute investigations in anatomy 

 and physiology prior to the vast improvements effected in the com- 

 pound microscope by the achromatisation of its object-glasses. 



Another form of simple magnifier, possessing certain advantages 

 over the ordinary double-convex lens, is that commonly known by 

 the name of the ' Coddington ' lens. 2 The first idea of it was given 

 by Dr. Wollaston, who proposed to apply two plano-convex or hemi- 

 spherical lenses by their plane side, with a ' stop ' interposed, the 

 central aperture of which should be equal to one-fifth of the focal 

 length. The great advantage of such a lens is, that the oblique 

 pencils pass, like the central ones, at right angles to the surface, so 

 that they are but little subject to aberration. The idea was, how- 

 ever, greatly improved upon by Sir D. Brewster, who pointed out 

 that the same end would be much better answered by taking a 

 sphere of glass, and grinding a deep groove in its equatorial part, 

 which should be then filled with opaque matter, so as to limit 

 the central aperture ; in other words, Brewster made Wollaston's 



r no-convex lenses hemispheres. Such a combination gives a 

 ge field of view, admits a considerable amount of light, and 

 is equally good in all directions ; but its power of definition 

 is by no means equal to that cf an achromatic lens, and its 

 working distance is inconveniently small. This form is chiefly 

 useful, therefore, as a hand-magnifier, in which neither high power 

 nor perfect definition is required, its peculiar qualities rendering 

 it superior to an ordinary lens for the class of objects for which 

 a hand-magnifier of medium power is required. Many of the 

 magnifiers sold as ' Coddington ' lenses, however, are not really 

 portions of spheres, but are manufactured out of ordinary double- 

 convex lenses, and are therefore destitute of the special advantages 

 of the real ' Coddington.' The ' Stanhope ' lens somewhat resembles 

 the preceding in appearance, but differs from it essentially in 

 properties. It is nothing more than a double-convex lens, having 

 two surfaces of unequal curvatures, separated from each other by a 



1 Transactions of the Society of Arts, vol. xlix. 



2 This name, however, is most inappropriate, since Mr. Coddington neither was, 

 nor ever claimed to be, the inventor of the mode of construction by which this lens 

 is distinguished. 



