ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 677 



hydrogen light living diatoms and sections of plants are extraordinarily 

 beautiful, all natural colours appearing very bright. With a power 

 of 2000 the cornea of a fly occupies the entire field of view, and the 

 fine vitreous membrane in each cell is seen magnificently." 

 Eock-sections can also be well shown in polarized light. 



Apparatus for Projecting an Image to any required Distance 

 with Variable Amplification* — For lectures it is often desired to 

 throw on the screen an image of an object with a given amplification, 

 and in order to vary the amplification, several auxiliary appliances 

 have hitherto been brought into requisition. A. Crova has devised a 

 means by which, with the same distance of the object from the screen, 

 the amplification may be changed with the aid of only one additional 

 piece of apparatus. 



He places between the object and the screen two lenses of equal 

 focus, one plano-convex and the other plano-concave, their distance 

 apart being capable of being altered as required. The plano-convex 

 lens is fixed in a frame which is fastened to a horizontal brass rod 

 resting on the stand, and along this rod the other lens (similarly fixed 

 in a frame) is made to move by rack and pinion. The lenses have a 

 focus of 0-15. By means of divisions marked on the rod the lenses 

 can be set at the distance required ; when the plano-concave lens is 

 at zero the two lenses are completely in contact, and their optical 

 centres coincide ; according to the distance between the lenses the 

 converging or diverging effect of the system predominates. 



Waechter's Travelling Dissecting Microscope. t — This, Fig. 121, 

 Fig. 121. * 



* Joura. de Physique, 1881, p. 159 (1 fig.) 



t Bericht wiss. Instrumente Berliner Gewerbeausstellung im Jahre 1879 (L. 

 Loewenberg, 1880) p. 302 (1 fig.). 



