THE MICROSCOPE AND ITS ACCESSORIES. 59 



movement. It is raised or lowered by a milled head on the 

 right-hand side of the stage, and when at its lowest position 

 a further turn of the milled head throws the whole sub-stage 

 out to the right-hand side, as illustrated, thus allowing the con- 

 denser to be easily removed or altered. The stand is sold with an 

 Abbe condenser, iris diaphragm, and two eye-pieces. 



W. Johnson and Sons. — Fig. lo illustrates Messrs. Johnson 

 and Son's Advanced Students' Microscope. The instrument has a 

 horse-shoe foot, the body being slung between two pillars. It has 

 a rack and pinion coarse adjustment, differential screw fine adjust- 

 ment, and a draw-tube, which is marked for the English and Con- 

 tinental objectives. The speciality in this instrument is the sub- 

 stage adjustment, the milled head. A, of which is let into the pillar 

 of the stand. This instrument was shown at the Royal Micro- 

 scopical Society last year, when the sub-stage adjustment was highly 

 commended by the late Mr. John Mayall, jun., who said that "it 

 seemed to him that Messrs. Johnson had undoubtedly ' scored i ' 

 by bringing out this screw-focussing adjustment for the sub-stage.'^ 

 A mournful point of interest is connected with this stand, it being 

 the last instrument that Mr. Mayall criticised publicly. The 

 sub-stage, which has screws for centring purposes, carries an Abbe 

 condenser, with iris diaphragm. The sub-stage is fixed to the 

 tail-piece by a bayonet fastening, and can be removed if occasion 

 requires (see B). 



Powell and Lealand.— For the dearer class of stands a 

 foremost place is occupied by Messrs. Powell and Lealand's No. 3 

 model, Fig. 11. This instrument is made on the same principle as 

 their famous No. i model. It is supported by a tripod stand. 

 The coarse adjustment is effected by a triangular bar, manipulated 

 by two large milled heads. At the top of the bar is the arm which 

 carries the body-tube. This arm also forms the case for their 

 delicate fine adjustment screw, which acts directly upon the nose- 

 piece ; the actuating milled head of the fine adjustment is seen at 

 the end of the arm. The stage has the ordinary rectangular and 

 sliding movements. The sub-stage is fitted with mechanical 

 centring movements, and has a rack and pinion coarse adjustment. 

 It will be noticed that the plano-convex mirrors are fitted upon a 

 double-jointed arm. 



