raVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 1057 



upon the stage of the instrument. It does more than this ; for it 

 enables the observer, when he has produced any appearance or eflfect by 

 the illumination which he desires to reproduce at pleasure, to register 

 the same, so that he can either resort with certainty to it at a future 

 time, or communicate the particulars to a friend, who, if possessed of 

 a similar instrument, can do likewise. 



"The subsidiary apparatus for illumination of a well-furnished 

 Microscope usually includes a set of achromatic condensers, the prism 

 of Amici, the parabola of Shadbolt, and Bergin's addition to the 

 latter for oblique illumination. It is unnecessary to go into any detail 

 of the trouble experienced, and the time frequently consumed in obtain- 

 ing, with the assistance of one or more of these appliances, a satis- 

 factory illumination. These di-awbacks are well known to micro- 

 scopists. For the information of others, I may state that frequently 

 five minutes of very eye-teasing work, and sometimes three times that, 

 are devoted to obtaining a satisfactory result, which, after all, is liable 

 to be undone by an incautious touch of the mounting, and which is 

 only to be restored by the same tentative process of the previous 

 adjustment. 



" It was such experiences as these which led to the improvements 

 combined in the present instrument. A little consideration was 

 sufficient to show that, assuming we are in possession of an illumina- 

 ting pencil of unexceptional quality for every kind of illumination 

 required, then every kind of such, including the illumination of 

 opaque objects, will be comprehended under two heads, viz. first, the 

 means of applying such illuminating pencil at all angles with 

 respect to the jilaue of the object (or the stage of the Microscope) ; 

 secondly, the means of applying the pencil at all azimuths of 

 same. 



" This generalization, so to speak, of the illumination indicated 

 the means of carrying it out effectually. I had previously ascertained, 

 from direct use, that an achromatized prism was capable of giving 

 every kind of illumination required, in a manner not surpassed by 

 other means extant. Itc^jecting the difficult matter of causing the illu- 

 minating pencil to move in azimuth round the object, I devised the 

 present stage, which, while it is made to revolve, has those objections 

 to revolving which appertain to other stages removed ; and, by making 

 a little variation in the manner of attaching the body (or tube) of the 

 instrument to its arm, means are provided for readily bringing the optic 

 axis of this tube to pass through the centre of revolution of the stage, 

 and thus all objection to revolving the object, instead of the light, is 

 got rid of. 



" For the other movement of the prism (or that vertical to the 

 plane of the stage), I have, as maybe seen, adopted a sector, on which 

 slides the carriage containing the prism and including the ordinary 

 adjustment for focussing and a small azimuthal movement for modi- 

 fying the illumination. This sector is attached to the same piece 

 which carries the stage, and so that its centre, if i)ro(lueod, would cut 

 the optic axis of the tube where an object mounte<l upon a glass slide 

 of the ordinary thickness and laid upon the stage of the instrument 



