I. MICROSCOPES. 777 



refracting material. The instrument measures to the 100,000th of an inch, 

 the thickness of a thin plate as covering glass 004" thick, and the distance 

 which an image is refracted upwards. 



A minute prism reflects the light through a small plano-convex lens fixed 

 at the end of the measuring screw ; the prism is illuminated by solar or arti- 

 ficial light by a condenser, and reflected up the axis of the microscope. Dif- 

 ferential toothed wheels measure the number of revolutions of the screw 

 (nearly 100 threads to the inch), and indicators give the lOOths, l,000ths, 

 10,000ths. and 100,000ths "of an inch. The instrument detected 32 

 changes of colour in Newton's rings of contact between the central black 

 spot in air, caused by a film half a millionth of an inch thick, and the last 

 vanishing colour on separating the plano-convex lens from a plane surface 

 with which it had been in contact. It has measured the refractive index to 

 three places of decimals in thin flint glass 0" 0042 thick. 



3616a. Sorby's Standard Interference Scale, for 



measuring the position of absorption -bands in spectra. 



R. $ J. Seek. 



3616b. Sorby's Volute Diaphragm, for regulating the 

 amount of light for the spectrum microscope. R. fy J. Beck. 



3617. Early Form of Machine, with Knife, for making 

 microscopical sections of wood, &c., devised by Andrew Pritchard, 

 F.R.S.E., prior to 1835. Dr. Urban Pritchard. 



The block of wood for section, if large enough, is fixed in the movable 

 triangular chamber by means of the little screw. The larger screw at the 

 bottom gradually elevates the whole, and the knife, held in both hands, shaves 

 off thin sections as the wood is raised. Should the piece of wood be too small 

 to be placed in the triangular chamber, it must be glued on to a block of 

 convenient size. 



The whole machine is made to screw on to a bench. 



3618. Apparatus for Maintaining an even Temperature 

 in Microscopic Observations. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



This instrument is intended for performing microscopic operations in a 

 perfectly even temperature by means of hot water circulation. A small spirit 

 lamp is placed under the reservoir at the extremity of the apparatus. A 

 thermometer placed inside the instrument serves to regulate the temperature. 



3619. Camera for Microscope. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



3620. Two Boss Compressors, for the Microscope. 



(Ordinary models.) 



Geneva Association for Constructing Scientific Instru- 

 ments. 



3621. Compressor, by Schick, for the Microscope. 



(Ordinary model.) 



Geneva Association for Constructing Scientific Instru- 

 ments. 



