ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



315 



contrary, that they usually follow this kind of demonstration with the 

 greatest interest. The room, also, need not be entirely dark ; gas may 

 be burnt perfectly well at one end, so that the master may control the 

 behaviour of his pupils." 



We are inclined to think that the value of the Sciopticon is over- 

 rated by Dr. Hallier, at least for natural objects. We recently 

 obtained one from Germany, but were warned by the maker that 

 whilst excellent for showing photographs, it was limited in its opera- 

 tion for ordinary preparations. The image cannot be made more 

 than a half metre in diameter in the latter case, as against 8-9 feet 

 in the former, as there is too little light.* The lime light would of 

 course allow of a larger image and higher objectives, but one of the 

 advantages of the apparatus is then lost. 



The instrument in its ordinary form is sufficiently familiar. With 



Fig. 64. 



the arrangement for microscopic slides it is shown in fig. 64, in 

 which a, h, c, d is the special addition (in place of the usual lenses) 

 necessary for projecting the image on a screen, a being the stage, 

 b the objective (25-30 mm. focal length), c a diaphragm, and d the 

 focusing arrangement ; p q are the condensing lenses, having a 

 blackened cone e in front of them. The part JiTi is movable for 

 adjusting the illumination, s f F is the lamp, A B the case for it, 

 closed by glass at G G, C the " chimney," and H a silvered reflector. 

 For the lamp is used petroleum in which camphor is dissolved 

 to saturation. A painted white wall is the most suitable for receiving 

 the image. If a transparent image is desired, very white tracing 

 linen is best, instead of wetted linen which is decidedly to be avoided. 



* M. Fritz, in 'Das Scioptikon vervollkommneter Projection sapparat fiir den 

 Unterricht,' 6th ed., 1881, vi. and 83 pp. (4 figs.), says "The images are quite 

 sharp and bright enough, if not exceeding 50 cm. in diameter, to be seen with all 

 details by eight to ten persons simultaneously." 



