IV. 11ADIOMETBUS. 193 



892a. Photometer, fitted with clock, governor, pressure gauge, 

 and all necessary apparatus complete, as adopted by the Govern- 

 ment of Canada. William Sugg. 



893. Photometer by Bun sen, simplified by Professor Bonn. 

 Physical Collection of the University of Giessen, Pro- 

 fessor Buff. 



The standard for comparison is a pure stearine candle of known weight. 

 The measure wound upon the cylinder serves to determine the distance at 

 which the oil spot on the paper, when viewed from the second flame, is 

 brought to disappear. First the standard caudle, and then the flame, to be 

 measured, are thus investigated. The intensities of the two lights are to one 

 another as the squares of their distances from the oil spot. 



894. Photometer, for ascertaining amount of daylight. 



Scottish Meteorological Society. 



The light is reduced by turning round the graduated milled head at the 

 side, which works simultaneously and by equal degrees the two shades which 

 thus reduce the area of the aperture. At the opposite end of the box a 

 printed page is looked at through the eye-piece till it ceases to be legible, 

 when the result is read off in revolutions of the milled-head. Designed by 

 Thomas Stevenson, C.E., F.R.S.E., Honorary Secretary, and described in 

 Society's Journal, vol. iii., page 292. 



895. Selenium Photometer. Siemens and Halske, Berlin. 



It being the property of selenium that its electrical resistance is diminished 

 by the action of light, the diminution being proportional to the intensity of 

 the light, this apparatus is constructed with a plate of selenium forming part 

 of an electric circuit which is brought by rotating the cylinder containing the 

 plate alternately under the action of a normal candle sliding on a scale and of 

 the light to be measured. The normal light is adjusted on the sliding scale 

 until the electrical resistance of the selenium remains constant under the 

 action of the two sources of light, and the intensity of the light to be mea- 

 sured is calculated from the relative distances of the lights from the selenium 

 plate. 



897. First Heliostat, invented by 'sGravesande. 



Professor Dr. P. L. Rijke, Lei/den. 



(See 'sGravesande's " Physices Elementa Mathematica, " ed. III., Tom. 

 I., page 715.) 'sGravesande was an eminent Dutch geometrician, b. 1688, 

 d. 1742. 



898. Heliostat, G. Johnstone Stoney's modification, made by 

 Spencer and Sons, Dublin, a cheap and useful form. 



Prof. W. F. Barrett, Royal College of Science for Ire- 

 land, Dublin. 



IV. RADIOMETERS. 



899. A Collection of Radiometers of different construc- 

 tion, with lamps and screen for making experiments. 



Professor Adolph Wcinhold, Chemnitz. 

 39508. N 



