492 



APPENDIX. 



a vertical direction, and reflect it to the axis of the body, C, 

 which carries the eye -pieces without any perceptible loss of 



Fig. 266. 



light. When an object is required to be very much heated, a 

 larger plate, the edges of which are heated by small spirit- 

 lamps, is laid upon the small plate, P. The magnifying 

 powers to be obtained in this microscope vary from 25 to 500 

 diameters, and all the apparatus necessary for the study of 

 mineralogy can be applied to it, such as the goniometer, 

 micrometer, &c. In short, in the general use of acids, 

 reagents, &c., no injury can happen to the lenses of the 

 object-glasses, as, being placed underneath, they are pro- 

 tected from the oxidizing vapours. 



MR. HETT S MICROSCOPES FOR INJECTIONS, ETC. 



The first of these instruments, represented at fig. 267, con- 

 sists of a slab of polished mahogany, of the form shown at a, d 

 a!, into which, beneath b, is fixed a metal pin, on which a brass 

 wheel revolves, carrying forty cells with their enclosed objects; 

 these are protected from dust, and the action of light by the 

 cover, b, V V, which is fastened to the slab, a, in such a manner 

 as to admit of being readily removed, a portion of the slab be- 

 tween d and d is cut away, as is also part of the cover between 

 b and V, so as to admit the finger, for the purpose of moving 



