INVERTED MICROSCOPE. 385 



The micrometer is seen in fig. 2, and consists of a brass mount- 

 ing 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 hundred 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 field 

 of the microscope; whereas, were it graduated longitudinally, 

 it 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 a 

 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 screwy it 

 is readily adjusted, and by pushing it backward or forward, 

 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, in the construction of his large 

 microscope. 



A NEW FORM OF GONIOMETER FOR MEASURING ANGLES OF CRYS- 

 TALS UNDER THE MICROSCOPE. 



The measurement of the angles of crystals beneath the 

 microscope is at best a very imperfect operation, for we can 

 only measure plane angles, the angles between the faces not 

 being measurable ; yet, imperfect as it is, it is an important 

 adjunct in certain researches, as may be seen by referring 

 to the following, which is an abstract from Lehmann's work 

 on Physiological Chemistry, where he speaks of detecting a 

 minute quantity of urea in albuminous fluids: "If the residue 

 of the fluids from which the coagulated matters have been 

 filtered be extracted with cold alcohol, and the solution rapidly 

 evaporated, so as to cause the chloride of sodium (taken up by 

 the cold alcohol) to separate as much as possible in crystals, 

 or then bringing a drop of the matter-liquid in contact with 

 nitric acid under the microscope, we shall observe the com- 

 mencement of the formation of rhombic octahedra, and the 



