18 Transactions of the Royal Microscojyical Society. 
be determined with sufficient accuracy for the use of practical 
petrologists, and they would then be able to study the micro- 
scopical structure of rocks in a more satisfactory manner than 
heretofore, and at once clear up a number of interesting questions, 
which, howeyer, relate more to geological and mineralogical pro- 
blems than to the construction and use of the microscope. I will 
not, therefore, further occupy your time in describing them, since 
I feel that I may have already made my address somewhat too 
special in my anxiety to point out a new direction in which the 
practical application of the microscope may be further developed. 
I must, however, not conclude without describing some appli- 
cations of my method of study in the case of organic structures. 
There can be no doubt that it will enable us to decide several 
interesting questions connected with the constitution of shells. If 
sections were prepared specially for this purpose the measurements 
might be made with abundant accuracy. However, in order to 
test what may be done, I have merely used sections cut many 
years ago to show the organic structure. Though they are only 
about tVo 0 of an inch thick they enabled me to learn many interest- 
ing facts. Thus, for example, in the case of a shell of Pinna, cut 
nearly parallel to the prisms, it was easy to see that when they are 
cut somewhat obliquely only one system of the lines of the grating 
is divided, the separation being in the line of the axes of the prisms. 
It was also easy to see that one of the images is nearly, if not 
quite, unifocal, and the other very decidedly bifocal. I obtained 
for the index of the ordinary ray I * 63, and for that of the extra- 
ordinary I *49. In the case of calcite these should be about I ’65 
and I • 48. 
In a section of the shell of Haliotis tiiberculata occur in the 
outer layer certain transparent portions, which one might readily 
suppose were only calcite. This method of study, however, 
enabled me to prove that they are really aragonite, since certain 
portions give two well-marked bifocal images. Comparing to- 
gether all the facts, they indicate that the substance has a powerful 
negative double refraction and two optic axes not much inclined 
to one another. The observed indices were I ’TI, 1*69, and I * 55, 
the mean being I * 65. That for aragonite is I * 63, which is as 
close an agreement as could be expected from one single set of 
measurements with a section only of an inch in thickness. 
These examples will, I trust, be sufficient to prove that it is 
possible even now to apply this method of study with success in 
‘the case of very thin sections. With improved apparatus and 
using sections specially prepared, there should be no difficulty in 
obtaining excellent results. 
