HARDWICKE'S SCIENCE-GOSSIP. 



THE MICROSCOPE AND MICROSCOPIC J 

 WORK. 



No. XL— By E. Kitton, E.R.M.S. 



THE comparatively low refractive power of glass 

 induced Dr. Goring to suggest the use of 

 •ewels in place of glass for the construction of 

 lenses for single microscopes, hoping to make that 

 form of instrument as perfect as reflecting or re- 

 fracting engiscopes ; " for single microscopes natur- 

 ally aplauatic, or, at least, sufficiently so for prac- 

 tical purposes, possess an incontestable superiority 

 over all others, and must be recognized as verging 

 oward the ultimatum of perfection in magnifiers. 

 The advantages obtained by the more improved 

 engiscopes resolve themselves into the attainment 

 of vision without aberration, and with considerable 

 angles of aperture ; but, against this must be set the 

 never-to-be-forgotten fact that they only show us a 

 picture of an object instead of nature itself. It re- 

 quires little knowledge of optics to be convinced 

 that this simple unadulterated view of an object 

 must enable us to penetrate further into its real 

 texture than we can hope to do by any artificial 

 arrangement whatever; it is like seeing an action 

 performed instead of a scenic representation of it." 

 The above passage shows what an imperfect instru- 

 ment the compound microscope, whether refracting 

 or reflecting, really was ; and Mr. Oldbuck was not 

 far wrong when he said that, its revelations were 

 not to be depended upon. The endeavours to form 

 lenses of a diamond were not successful. The 

 first diamond Mr. Pritchard operated upon was 

 lost, before he had quite finished it, and he says, 

 " my only consolation was to discover that, had it 

 been completed, its thickness and enormous refrac- 

 tive power would probably have caused the focus to 

 fall within the substance of the stone." His next 

 attempt was with a rose-diamond : this he deter- 

 mined to make into a plano-convex lens, and on 

 the 1st of December, 1824, he had the pleasure of 

 first looking through a diamond-microscope. Mr. 

 Pritchard goes on to say that " after I had polished 

 it sufficiently to put it into the hands of Dr. Goring, 

 who tried its performance on various objects, both 

 as a single microscope and as the objective of a 

 compound, he stated in a letter to me, dated the 

 '3rd of January, 1825, that 'it had shown the 

 most difficult transparent objects I have submitted 

 to it,' and again ' I can clearly perceive the amazing 

 superiority it willpossess when completely finished.' " 

 (It ought to be mentioned that at that time the 

 scales of the Podura were unknown as test objects.) 

 The lens was, however, never finished, as on further 

 polishing, several flaws were detected. After a 

 year's delay, he resumed his attempt to make a 

 perfect diamond lens, and succeeded in obtaining 



one without a flaw ; but another obstacle to its 

 correct performance now made its appearance, — it 

 proved to be doubly refractive, giving two images, 

 instead of one. 



Mr. Pritchard was, however, enabled to over- 

 come this difficulty, and at last succeeded in com- 

 pleting two perfect plano-convex lenses, one of the 

 twentieth of an inch and the thirteenth of an inch 

 focus ; of these the Duke of Buckingham became 

 the possessor. He afterwards made lenses of 

 sapphires, rubies, and garnets. Eig. 149 is given 

 by Mr. Pritchard to illustrate the longitudinal 

 aberration of glass and diamond lenses of the same 

 foci. 



Fig. 149. Diagram showing lon- 

 gitudinal aberration of glass 

 and diamond lenses. 



Fig. 150. Diagram showing 

 respective curvatures of a 

 sapphire and glass lens. 



G represents the section of a semi-lens of glass ; 

 D, one of diamond. They are so placed that the 

 principal focus, E, in each of them shall fall on the 

 same point. The marginal rays will intersect the 

 axis at d in the diamond and g in the glass, and the 

 breadth of the space from (/ to E will be the longi- 

 tudinal aberration of the diamond lens, and the 

 space from g to E the longitudinal aberration of 

 the glass lens. 



The refractive indices of the diamond, garnet, 

 ruby, and sapphire, and glass, are respectively 

 2-5, 1-8, and 1*5. 



Great increase of magnifying power is, of course, 

 obtained by the great refractive properties of 

 jewels. A diamond ground to the same curves 

 as would be required to form a glass lens of ¥ V 

 of an inch focus equal to 800 diameters would 

 magnify 2,133 diameters, and would be equal to 



a glass lens of 



of an inch focal length. A 



sapphire or ruby lens, with the same curvatures, 

 would amplify 1,333 diameters, and be equal to a 

 glass lens of rhj of an inch focal length. Eig, 

 150 represents the respective curvatures of a 

 sapphire and glass lens, the upper half repre- 

 senting the sapphire, and the lower the glass lens, 

 E being the focus. Lenses formed of gems possess 

 a still further advantage, viz., their imperishability, 

 the surface of glass lenses becoming oxidized in the 

 course of time ; but with all these advantages the 

 jewel lens is a thing of the past, although, from its 

 high refractive and low dispersive power, it was 

 more nearly aplanatic than; one of glass, it can 



