W. A. Miller on Photographic Transparency, etc. 105 
from three-fifths to four-fifths or even more of its length. Mica produ- 
ces a similar effect. Indeed, the only substance which the author found 
could be employed with advantage i is rock-erystal cut into thin slices and 
om isbed. The value of this material in researches upon the more refrangi- 
I 
querel several years a In order to hold the liquids for experiment, a 
small trough was prepared by cutting a notch ina thick plate of plate- 
glass, the sides being completed by means of thin plates of quartz, which 
were pressed against the ground surfaces of the plate-glass by the aid of 
elastic bands of caoutchouc; a a of liquid of 0°75 inch in depth 
was thus obtained for each experim 
The substances which, after assume air and certain other ; 
are most perfectly diactinic, are rock-crystal, ice, as well as pure water, 
and white fluor-spar. Rock-salt is scarcely inferior to them, if at all. 
Then follow various sulphates, including those of baryta, and the hydra- 
ted sulphates of lime and magnesia, as well as those of the alkalies. The 
carbonates of the alkalies and alkaline earths, as also the phosphates, ar- 
seniates, and borates, are likewise tolerably transparent, though satura 
solutions of phosphoric and arsenic acids exerted considerable absorbent 
power; so also did those of the alkalies, potash, and soda, possibly from 
the presence of a trace of some foreign coloring matter, as those quid 
had an extremely faint greenish tinge. 
soluble fluorids, as well as the chlorids and bromids of the metals 
of the alkalies and alkaline earths, are freely diactinic, but the iodids are 
much Jess so, and exhibit certain ae All the a acids and 
their salts which were tried by the author exerted a marke rbent 
however, much more difficult to obtain organic compounds in a state 
of purity sufficient to furnish trustworthy results, than is the case with 
the salts of the inorganic acids. The author, therefore, expresses himself 
with more reserve upon some of setae organic bodies, Pees the 
acetates, than in other cases. The different varieties of sugar freely 
diactinic. : 
Amongst the salts of i inorganic acids, the nitrates are the — remark- 
able for their power of arresting the chemical rays. A solution of each 
of these salts, in all the instances tried, cut off all the more ‘sabeatigible 
rays, and reine the spectrum to loss than a sixth of its ordinary 
length. The chlorates, ss do not participate in this absorptive 
saree to nearly the same 
Although the mire ray asa lass, are largely srvenne’ = ~~ 
are much less so; and the hyposulphites cut off about 
the length of the spectrum, Eoriet only the less re age. meer 
eighteen different liquids tried by the author, two only can be re- 
garded as tolerably diactinic, viz: water, which is eminently so, and abso- 
hol, —— however, exhibits a considerable falling: off The 
tie wh hich fol —— in the order of their chemical trans- 
most trans t being mentioned first :— Dutch liquid, 
PPO ites ether; then eae on distilled glycerin, which b dir but 
Am. Joun. Bok—-Racox Ssrizs, Vou. XXXVI, No. 106.—Juxy, 1 
14 
