PHOTOMETKY. 131 



century, have been added to the stars in the generally con- 

 sulted Uranometria Bayeri, are not, as was long supposed, 

 certain indications of these alterations of light. Argelander 

 has ably shown, that the relative brightness of the stars cannot 



placing on one another two lenses of a great focal length, co- 

 loured rings will be formed round their point of contact as much 

 by means of reflection as of transmission. The colours of the 

 reflected rings are complementary to those of the transmitted 

 rings ; these two series of rings neutralise one another when 

 the two lights by which they are formed and which fall 

 simultaneously on the two lenses are equal. 



" In the contrary case, we meet with traces of reflected or 

 transmitted rings, according as the light by which the former 

 are produced, is stronger or fainter than that from which 

 the latter are formed. It is only in this manner that co- 

 loured rings can be said to come into play in those photo- 

 metric measurements to which I have directed my attention." 

 (b.) Cyanometer. 



" My cyanometer is an extension of my polariscope. This 

 latter instrument, as you know, consists of a tube closed at one 

 end by a plate of rock crystal, cut perpendicular to its axis, 

 and 5 millimetres in thickness ; and of a double refracting 

 prism placed near the part to which the eye is applied. Among 

 the varied colours yielded by this apparatus, when it is 

 traversed by polarised light and the prism turns on itself, we 

 fortunately find a shade of azure. This blue, which is very 

 faint, that is to say mixed with a large quantity of white when 

 the light is almost neutral, gradually increases in intensity in 

 proportion to the quantity of polarised rays which enter the 

 instrument. 



" Let us suppose the polariscope directed towards a sheet 

 of white paper ; and that between this paper and the plate 

 of rock crystal there is a pile of glass plates capable of being 

 variously inclined, by which means the illuminating light of 

 the paper would be more or less polarised ; the blue colour 

 yielded by the instrument will go on increasing with the in- 

 clination of the pile ; and the process must be continued until 

 the colour appears of the same intensity with the region 

 of the atmosphere whose cyanometrical tinge is to be deter- 



K2 



