A Mode of altering the Focus of a Microscope. By M.Govi. 205 
planes, and interspersed are numbers of very minute and delicate 
cuneiform crystals.* It has also a small cloud of exceedingly 
small cavities. 
Another remarkable specimen may be mentioned here, which 
has small cavities and minute microscopic crystals. It is of a pale 
yellow or straw colour, and of a depth and brilliancy scarcely 
exceeded by the diamond. 
During the examination, about two years since, of some hun- 
dreds of small crystals of sapphire, perfectly transparent to dark 
blue, I discovered one which had very singular plumose impressions 
on the planes of the prism. This induced me to examine carefully 
all those which I subsequently procured, and I have now over 
a dozen specimens which exhibit this very singular character. I 
am entirely at a loss to discover the cause of this form of minute 
impressions on so hard a substance. It evidently has been formed 
by some collateral mineral substance, against which the molecules 
in crystallization have been arranged. 
Diamond.—The hardest of all substances stands first among 
gems. It has not, however, much interest to the microscopist, as 
no cavities with fluid have been, so far as known, observed; nor 
has it included crystals of foreign substances. They are often very 
imperfect, containing rifts and discolorations. Some of my speci- 
mens have beautiful triangular impressions on the surface of the 
planes. My friend Dr. Hamlin, of Bangor, Maine, is engaged on 
an extended work on the diamond. Such a work is much needed, 
and I know no one as capable as he to accomplish it. This gem 
sometimes occurs of various colours. In my cabinet I have six 
different colours.—Proceedings of the Academy of Natural 
Sciences, Philadelphia, 
VI—A Mode of altering the Focus of a Microscope without 
altering the Position of either the Objective or the Object. 
By M. Govt. 
In working with the microscope, above all when we utilize it for 
comparing measures of length, it often happens that after having 
focussed it upon the first object, it is necessary to apply it to the 
observation of a second one, which is not absolutely at the same 
distance from the objective as the first. Then it becomes necessary 
to alter the focus of the instrument in order to have a clear image 
of the object, and this is done either by a movement of the whole 
microscope (i. e. the tube), or by an alteration of the eye-piece or the 
objective, or by a movement forwards or backwards of an inter- 
* Proc. Acad, Nat. Sci” May 11, 1869. 
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