240 J. Lawrence Smith on the Inverted Microscope. 
correspond with the opening at g, and at the same time there is 
an opening in their mounting which is made to come opposite to 
that of g. The micrometer is seen in fig. 2, and consists of a 
brass mounting B with a small plate of glass A, having near the 
outer edge a fine graduate scale (the one used is ten millimetres 
divided in one hnndred parts) made in the direction of the breadth 
and not of the length of the micrometer, which little circumstance 
_ is of vast importance : for made as it is, it can sweep the fiel of 
the microscope; whereas were it graduated longitudinally, 1t 
would simply move in the radii of the field, and therefore could 
not be brought on the object in many of its positions. 
The manner of using the micrometer can be understood in 4 
few words. In examining with any eye-piece, if it be required 
to measure an object. The micrometer B is introduced into the 
opening g, and if not seen distinctly, by turning the screw p 1t 1s 
readily adjusted, and by pushing it backwards and forwards, or 
turning the tube D, the graduated scale can be readily brought 
over the image of the object, either longitudinally or otherwise ; 
and knowing the value of each division, the dimensions of the 
object is readily made out. The manner of ascertaining the value 
of these divisions is learnt in almost every work on the microscope. 
This method of mine is now adopted by M. Nachet, of Paris, 1 
the construction of his large microscope. 
A new form of Goniometer for Measuring Angles of Crystals 
under the Microscope.—The measurement of the angles of crys 
tals beneath the microscope is at best a very imperfect operation, 
for we can only measure plane angles, the angles between - 
faces not being measurable ; yet, imperfect as it is, it 1s aa 1m" 
portant adjunct in certain researches, as may be seen by referring 
to the following, which is an extract from Lehmann’s work on 
Physiological Chemistry, where he speaks of detecting @ minute 
quantity of urea in albuminous fluids. 
“Tf the residue of the fluid, from which the coagulated mat- 
ters have been filtered, be extracted with cold alcohol, and the 
solution rapidly evaporated, so as to cause the chlorid of s 
Sead 
m 
hexagonal tablets, in which, if the investigation is to be unques- 
tionable the acute angles (=82°) must be always measured, After 
the determination of the nitrate, we may also obtain the oxalate, 
and submit it to microscopic examination. A good eryst@ line 
determination yields the same certainty as an elementary analysts; 
which, in these cases, would never or extremely seldom be possible. 
