Transparent onhj to Ultra-Violet Light. 259 



eventually removed by means of a thin sheet of Chance's 

 " signal-green "" glass, such as is used for one of the reflectors 

 in the Ives Kromskop. 



This combination is wholly opaque to visible light, while 

 freely transmitting everything between wave-lengths 34 and 

 38. Of course the employment of glass screens limits the 

 ultra-violet transmission, and a screen of this description is 

 useful chiefly for lecture demonstrations. Considerable care 

 must be used in the adjustment of the strength of the solution 

 of the nitroso in gelatine, otherwise the intensity of the ultra- 

 violet light is considerably weakened. 



The best strength is such as will be just sufficient to remove 

 the blue and violet light transmitted by dense cobalt glass. 

 Quite a number of trials will be found necessary in adjusting 

 the densities of the three components of the screen to secure 

 the maximum effect, but when the balance is just right, it is 

 possible to form a focus in which a piece of paper is quite 

 invisible, while a mass of crystals of the nitrate of uranium 

 (which I have found superior to anything else) glows with 

 sufficient intensity to be seen from the back of the largest 

 lecture-room. 



It is best to exclude carefully all light which does not pass 

 through the screen. 



With the assistance of one of our students, lam at the 

 present time investigating the absorption of a large number 

 of substances, which, so far as I know, have not been pre- 

 viously studied, and I hope in time to dispense with glass 

 entirely, and produce an opaque screen which transmits ultra- 

 violet down to the end of the spectrum. 



A combination of a tube furnished with quartz ends, on one 

 of which is a thin film stained with nitroso, transmits all of 

 the ultra-violet, and only the extreme red, but it is very incon- 

 venient to work with. For use as a screen in spectrum- 

 photography there is no especial object in removing the red, 

 yellow, and green, the nitroso alone blocking out completely 

 the actinic portions of the visible spectrum, which overlie the 

 ultra-violet in the second and third order spectra, and I shall 

 next consider solutions of the substance in various fluids in 

 connexion with spectrum -photography. 



I have found that the best method of quickly securing a 

 record of the absorption of a solution is to bring a prismatic 

 layer of the liquid, contained in a quartz cell, before the slir 

 of a quartz spectrograph, and photograph the spectrum of the 

 cadmium spark. We secure in this way a record o£ th? 

 absorption of the liquid in various thicknesseSj in the form of 



S 2 



