14 
Quarterly Journal. 
light, and as regards the difference of colour, upon the; 
absorption of those undulations which are of some certain 
-wave length. Thus the emergent ray would only show that 
colour resulting from the compounding of those rays which 
had escaped absorption. As the elasticity of douWy-re- 
fracting minerals differs in certain directions as regards 
l'ght undulations traversing it, we can see that the light 
emerging in these directions will have been unequally 
absorbed, and will, therefore, differ in colour. 
We must further remember that a ray of polarised light 
passing through a plate of some suitable mineral, is resolved 
into two polarised rays, vibrating in the directions of the 
greatest and the least elasticity. If we were to examine 
such a plate slowly rotated over the polariser — the analyser 
being removed—we should observe that in two positions, 
perpendicular to each other, the light passing through 
assumed two different colours or tints. Thus we should be 
enabled to observe in sequence the two colours which 
together give the tint proper to the plate ; and we should 
further be able to distinguish between mineral species, such 
as hornblende, which are distinctly dichroic under such con¬ 
ditions, and other mineral species, such as augite, which 
are not. 
These statements are a brief and somewhat imperfect out¬ 
line of some of the methods nowin use in micro-petrography. 
Those who are desirous of obtaining a further insight into 
this most interesting and important branch of microscopical 
research may consult a little work lately published by Dr. 
Spottiswoode on “ Polarisation the admirable “ Mikro- 
skopische Physiographic der Petrograpliisch Wichtigen Mine- 
ralien” of Rosenbusch, or the more special optical studies of 
the “ Lehrbuch der Physikalischen Mineralogie” of Dr. 
Sclirauf. _ _ 
Notes on Sponge from Northern Territory. By F. Barxard. 
[With Plate.] 
[Read 26th July, 1876.] 
At the last meeting of this Society it will be remembered 
several natural history specimens, collected by Dr. Sturt in 
the Northern Territory and kindly sent by him for the use 
of the members, were distributed. Among them were 
three varieties of sponges, to one of which I would more 
particularly direct attention. 
