I. MICROSCOPES. 919 



and itself previously compared with a standard scale. The accuracy obtatned 

 is proportional to the square root of the number of divisions compared one 

 by one, and can be relied on to the y^Vry P ar * f a millimetre. 



The instrument should be placed before a window, and at a distance not 

 exceeding six feet from it, because otherwise the scale would not receive 

 sufficient light for seeing the divisions clearly through the microscope. 



A description of verifications made by the instrument, with cut, accom- 

 panies it. 



3608. Apparatus for Microscopical Research in the 



open air. Prof. Dr. Leonard Roesler, Klosterneuburg. 



3609. Re-agents for Micro chemical Researches, in box. 



Prof. Dr. Leonard Rooster, Klostcrneuburg. 



3610. Instruments for Microscopical Researches. 



Prof. Dr. Leonard Roesler -, Klosterneuburg. 



3611. Instruments for Microscopical Researches on 



living organisms, adapted to their observation when under the 

 influence of various gases and at different temperatures. 



Prof. Dr. Leonard Roesler, Klostcrneuburg. 



3612. Drawings illustrating preceding Apparatus. 



Prof. Dr. Leonard Roesler, Klosterneuburg. 



3615. Kratometer for finding magnifying power and focal 

 length of objectives. Dr. Royston Pigott, F.R.S. 



This simple instrument is so contrived that whatever object glass is used, 

 the actual power is ascertained at once, whatever be the length of body em- 

 ployed. A stage micrometer of lines ruled to lOOths and l,000ths of an 

 inch is viewed by the kratometer eyepiece, and the number of divisions of the 

 stage micrometer embraced by ten of the kratometer, gives exactly the 

 magnifying power, if multiplied by ten. This instrument at once determines 

 the focal length and comparative powers of all object glasses submitted to 

 this test. 



3616. Microscopic Refractometer, for ascertaining the 

 mean refractive index of plates of glass or lenses. 



Dr. Royston-Pigott, F.R.S. 



A new refractometer for determining the refractive index of white light 

 (or mean rays or line E in the solar spectrum) of small plates or lenses of 

 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,COOths '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. 



