354 SEC. 11. ASTRONOMY. 



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

 together by the same clockwork (which has been left behind), 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 answers per- 

 fectly 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. 



1809. Astrometer for Reflecting Telescopes, invented 

 by the contributor. E. B. Knobel, F.R.A.S., F.G.S. 



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 tivice 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 instru- 

 ment depending on the mathematical principle sin 30 = ^ the aperture is thus 

 always accurately equilateral, and concentric with the mirror or object glass. 

 The graduated base and the micrometer head give the side of the triangle, 

 whence the aperture is readily obtained. 



1810. Astro-Photometer, according to Glan's system. 



Schmidt and ffacnsch, Berlin. 



III. OBJECTS ILLUSTRATING THE HISTORY OF 

 THE TELESCOPE AND ASTRONOMICAL OBSER- 

 VATION. 



1811. Incomplete Telescope with broken lens of Galileo. 



Royal Institute of" Studii Superiori" Florence. 



1812. Compass of Galileo. 



Royal Institute of " Studii Superiori" Florence. 



1813. Magnet of Galileo. 



Royal Institute of " Studii Superiori "^Florence. 



1814. Telescope of Galileo. 



Royal Institute of" Studii Superiori" Florence. 



1815. Object Glass (broken) of Galileo. 



Royal Institute of " Studii Superiori" Florence. 



