406 SEC. 11. ASTRONOMY. 



180S. 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 of Rosse, F.R.S. 



The accuracy of the work done with this instrument depends on the 

 fact that, though the total light emitted by the flame varies with its 

 size, the intensity of the brightest part is appreciably constant. 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 

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

 eyepiece 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 to 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 in. aperture and 4 ft. focal length, the other of 

 1-2 in. aperture and 2 ft. focal length, are parallactically mounted and moved 

 together by the same clockwork (which is not exhibited), so that the 

 images of the two stars keep constantly the same place in the field during 

 observation. Being worked out in all parts with greatest care and on sound 

 optical principles, it can hardly be doubted that this instrument perfectly 

 answers its purpose ; but on account of the great number of constants to be 

 determined for it, its use is rather difficult. Until now only two of these 

 instruments have been practically applied, that of Schwerd himself, and the 

 one constructed for Wilna. In both cases the first problem has been the 

 determination of the co-efficient of extinction of light by the atmosphere of 

 the earth. 



18O9. Astrometer for Reflecting Telescopes, invented 

 by the contributor. E. B. Knobel. 



This instrument has been invented for determining the magnitudes of stars 

 on the principle of limiting apertures. It consists of an equilateral triangular 

 aperture, constructed of two plates, one forming the base and the other the 

 opposite angle of the triangle, connected by a screw shaft of peculiar con- 

 struction. The upper portion carrying the angle plate, being a right handed, 

 and the lower connected to the base plate, a left handed screw. The pitch 

 of the upper screw is twice that of the lower. By simply turning the milled 

 head at the end of the shaft, the aperture is made smaller or larger within the 

 limits of the triangle inscribed in the telescope tube, and zero. The iustru- 



