20 THE MICROSCOPE AND ITS REVELATIONS. 



work done by means of them surprises every one who studies the his- 

 tory of Microscopic inquiry. An important improvement on the single 

 lens was introduced by Dr. Wollaston, who devised the doublet, still 

 known by his name; which consists of two plano-convex lenses, whose 

 focal lengths are in the proportion of one to three, or nearly so, having 

 their convex sides directed towards the eye, and the lens of shortest focal 

 length nearest the object. In Dr. W.'s original combination, no perfo- 

 rated 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 combina- 

 tions of this kind are well constructed, both the spherical and the chro- 

 matic aberrations are so much reduced, that the angle of aperature 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 sin- 

 gle lenses, among Microscopists (in this country at least) who were pro- 

 secuting minute investigations in Anatomy and Physiology prior to the 

 vast improvements effected in the Compound Microscope by the achro- 

 matization of its object-glasses ( 15); and, in particular, the admirable 

 researches of Dr. Sharpey, 2 on ciliary action in Animals (1830-35), and 

 Mr. Henry Slack's beautiful dissections of the elementary tissues of 

 Plants, and also his excellent observations on Vegetable cyclosis (1831), 3 

 were made by their means. The performance of even the best of these 

 forms of Simple microscope, however, is so far inferior to that of a good 

 Compound microscope, as now constructed, that no one who has the 

 command of the latter form of instrument would ever use the higher 

 powers of the former. And as it is for the prosecution of observations, 

 .and for the carrying on of dissections, which only require low powers, 

 .that the Simple microscope is chiefly needed, the Wollaston doublet has 

 now almost gone out of use. 



* 24. Another form of Simple magnifier, possessing certain advantages 

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

 name of the ' Coddington ' lens. 4 The first idea of it was given by Dr. 

 Wollaston, who proposed to apply two plano-convex or hemispherical 

 lenses by their plane sides, with a ' stop ' interposed, the central aperture 

 of which should be equal to one-fifth of the focal length. The great ad- 

 vantage 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 further 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. Such a lens gives a large field of view, admits a con- 



1 " Transactions of the Society of Arts," Vol. xlix. 



2 See his article Cilia in the " Cyclopaedia of Anatomy and Physiology," and 

 the references under that head in the Index to the present work. 



3 See his Memoir, with two beautiful Plates, in the ' ' Transactions of the Soci- 

 ety of Arts," Vol. xlix., pp. 6, 7. 



4 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. 



