280 Transactions of the Society. 



rather diminished in intensity. I also find that styrax, in which 

 the Naricula was mounted, is completely opaque to the second 

 baud, whilst balsam transmits it quite undimmed. The defective 

 definition would, therefore, be much greater with a balsam mount. 

 Monobromide of naphthalin is perfectly transparent to both bands, 

 so is cedar oil ; but realgar is unfortunately opaque to both, and 

 to the visible violet as well. The silica of diatoms is, of course, 

 very transparent to all the ultra-violet rays. 



A solution of methylen-blue is much more transparent to the 

 second band than the ammonia-copper, but has the disadvantage 

 of also transmitting the red. 



