226 E. M. NELSON ON HISTORIC MICROSCOPY. 



proved the Wilson microscope. Adams now improved his micro- 

 scope by making the body move at the junction between the arm 

 and the square bar, so that it could pass over any part of the object, 

 instead of moving the object under the body. This so-called im- 

 provement was eminently in a retrograde direction, as it threw the 

 optic axis of the body out of centre with that of the condenser, 

 but Boss adopted it, as we shall see presently. 



Adams put the condenser on a separate sub-stage, and altered 

 the sliding strip of powers to a wheel of powers ; in other words, 

 a rotating nose-piece. 



The last one of this series is the Martin-Jones. The following 

 were its main features : — Inclining ; rack-work focussing stage ; 

 sub-stage ; tripod stand replacing the usual box ; rotating nose- 

 piece ; draw-tube to body ; rack and pinion movement to body 

 in a vertical direction over the object ; tangential movement 

 to stage under the body, so the usual rectangular movements to 

 the object were given half by the body and half by the stage. 

 There was a super-stage bull's-eye condenser and Lieberkiihn. 



Martin after this brought out a microscopic pocket telescope. The 

 four-lens eye-piece of a small pocket telescope was made into a micro- 

 scope. The way this was carried out is so neat that it is worthy of 

 notice. By twisting one of the tubes over the other, an aperture 

 was revealed in the side of the telescope. The mirror, which stowed 

 away in the cap of the telescope, fitted into the tube and light fell 

 on it through the hole in the side. Objects were placed in small 

 circular holes formed round metal discs, which also packed away in 

 the cap. This formed what we should now call a rotating object- 

 holder. 



In 1777 De la Barre introduced changing eye-pieces, and in 1787, 

 under the second Adams, non-aplanatic microscopes may be said to 

 have reached their zenith. 



The next thing which engaged men's minds was to get better 

 results than the compound chromatic microscope would give. In 

 this country Wollaston worked at simple microscopes, and Amici, 

 on the Continent, at reflectors. 



In 1812 Wollaston brought out a periscopic doublet, and after- 

 wards Sir David Brewster an oil-immersion lens, an endomersion 

 objective, and lenses of gems. Then came Sir David Brewster's 

 great discovery, viz., the grooved sphere, which is the Oocluington 

 lens. Sir J. Herschel invented his doublet of a bi-convex and 



