270 PHYSICS. 



as usual. With it, this spectrum appears in a field of light, the bril- 

 liancy of which can be varied at pleasure. When the extinguishing 

 flame is distant, the whole spectrum is visible. As it comes nearer, the 

 violet first disappears, and then the other colors in the order of their ' 

 brightness. Though the j-ellow is commonly supposed to be the bright- 

 est of the rays, yet it disappears in this treatment in the order of its 

 refrangibility, the red being plainly perceptible long after the yellow has 

 gone. To render the extinguishing light more intense in order that it 

 may obliterate very bright spectra, the simple and efficient device was 

 adopted of depositing an exceedingly thin film of silver on the reflect- 

 ing face of the prism. This, while not perceptibly diminishing the 

 amount of transmitted light, increased markedly that of the reflected 

 beam. Sunlight was then tried w>th the same results. The prism was 

 then replaced by a grating, and the experiment repeated on the diffrac- 

 tion spectrum. In this all the colored spaces yielded apparently in an 

 equal manner, and all disai^iieared at the same moment. On diminish- 

 ing the illumiiiation, all the colors came into view apparently at the same 

 time. The author concludes : 1st, that in the prismatic spectrum the 

 luminous intensity increases from the more to the less refrangible spaces, 

 it« maximum being noc in the yellow, but iu the red. This is due to the 

 action of the prism, which narrows and as it were condenses the colored 

 spaces more and more as we pass toward the red, increasing the intensity 

 of the light as it does that of the heat; 2d, that in the grating or dif- 

 fraction spectrum the luminous intensity is equal in all the visible regions, 

 all the colors being simultaneously obliterated by an extinguishing light. 

 The distribution of heat is therefore that of light. — [Am. J. Sci., Ill, 

 xviii, 30, July, 1879.) 



Cornu has contrived a spectroscope intended especially for the study 

 of the ultra violet rays. The object-glass alike of the collimator and the 

 observing telescope is an achromatic lens, consisting of a double convex 

 lens of quartz and a plano-concave lens of calc-spar, both cut ])erpen- 

 dicular to the optic axis. This combination is esi)ecially useful for the 

 ultra violet rays, allowing all the rays to pass that reach us through 

 our atmosphere, and being almost absolutely achromatic for them ; so 

 that the adjustment required for different parts of the spectrum is 

 almost nothing. The prism used may be of calc spar, though it absorbs 

 rays of the highest refrangibility ; or of quartz, though if the axis is 

 parallel to the optic axis, the double refraction interferes. This is obvi- 

 ated by using a prism made of two halves of contrary rotation, the 

 interior face being perpendicular to the axis. To obtain a double dis- 

 persion, the two prisms of quartz have their bisecting planes perpen- 

 dicular to the optic axis, but having opposite rotatory power. — {J. Fhys., 

 viii, 85, June, 1879.) 



Lamansky has proposed a special form of spectroscope for studying 

 fluorescent liquids. Ordinarily a glass plate has intervened between 

 the liquid and the spectrum made to enter it. Hagenbach places the 



