II. MOLECULAR STRUCTURE. 353 



1806c. Solar Spectroscope, used since 1868 in observing 

 solar phenomena, made by Browning. 



J. Norman Lockyer, F.R.S. 



1806d. Slit arrangements for Spectroscopes. 



J. Norman Lockyer. 



d. STELLAR PHOTOMETRY. 



1807. Astronomical Photometer for Extinction. De- 

 signed by Professor Thury. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



The apparent brightness of 'a heavenly body seen in the telescope is gra- 

 dually reduced by the changeable diaphragm placed before the objective, and 

 if necessary by the interposition of one or two dark mirrors placed behind the 

 eye-piece in the square box, which is exposed with the diaphragm. The light 

 is gradually reduced until the body is no longer visible. The aperture of the 

 diaphragm is then shown upon a dial placed under the eye of the observer. 



The full description of this apparatus is to be found in the " Archives des 

 <: Sciences physiques et naturelles de Geneve," 1874. 



1808. Zollner's Astrophotometer, for measuring the light 

 of the heavenly bodies by comparison with that emitted by the 

 brightest portion of the flame of a paraffin lamp. 



Earl ofRosse, F.R.S. 



It being found that, though the total light emitted by the flame varies with 

 its size, the intensity of the brightest part does not, appreciably. Two 

 artificial stars are formed by means of a pin hole, a double concave lens, and 

 a double convex lens, which appear in the field by reflexion from front and 

 back faces of a plate of glass alongside of the image of the real star whose 

 light passes through the plate. The intensity of the artificial star is varied, 

 first by changing the pin hole, and finally by two Nicol's prisms, the colour 

 being first matched with that of the star by means of a third Nicol, with a 

 quartz plate between it and the first of the other two Nicols. The instru- 

 ment is provided with object glasses of various sizes (and diaphragms) up to 

 2f inches, and, if fainter stars are to be examined, can be screwed on to the 

 eye-piece of an equatorial instrument. A second arrangement, like the first, 

 but without the quartz plate arrangement, forms an artificial star from moon- 

 light, for comparison of the light of that body with the artificial star. 



896. Photometer, constructed by Schwerd for the Observa- 

 tory of Pulkowa. The Imperial Observatory, Pulkowa. 



In agreement with Prof. Argelander and M. Otto Struve, the late Prof. 

 Schwerd of Speyer constructed, in 1863, four photometers of the same size, 

 two for Russia (Pulkowa and Wilna), the third for the Observatory, Bonn, 

 the fourth for his own use. The principle of the construction is that of com- 

 paring the light of different stars exhibited in the same field by telescopes of 

 different aperture. The diameter of the diaphragms to be applied before the 

 two object-glasses, and corresponding systems of lenses, for purpose of pro- 

 ducing equal light and colour, gives the measure of the relative brightness. 

 The two telescopes, one of 2-3' aperture and 4 ft. focal length, the other of 



39508. Z 



