386 INVERTED MICROSCOPE. 



hexagonal tablets, in which, if the investigation is to be un- 

 questionable, 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 

 crystalline determination yields the same certainty as an elementary 

 analysis, which in these cases would never, or extremely seldom, be 

 possible." 



Two of the best goniometers used for this purpose are those 

 of Eoss and Leesom, the latter being doubtless the best, and 

 based on the use of the double refracting spar. After trial, 

 however, I find it neither as accurate nor as economical in its 

 construction as the following : Around the tube 1ST (fig. 1) there 

 is a collar X fastened to it. On the collar there is a graduated 

 circle S, about three inches in diameter, turning freely on X. 

 On the tube G- a small index t is fastened. These are all the 

 additional parts necessary, as the micrometer just described 

 is used to aid in the measurement, which is accomplished as 

 follows : The angle of the crystal to be measured is brought 

 as near the center of the field as the eye can readily judge of 

 (a little deviation will not sensibly affect the measurement). 

 The micrometer is then introduced in the opening g, and 

 turned about until the lines are parallel to one side of the 

 angle, or until one of the long marks corresponds with that 

 side. This done, without disturbing the tube G, the graduated 

 circle S is turned until the index t points to zero. Now look 

 again into the instrument, and turn G until the markings on 

 the micrometer are parallel with the other side of the angle; 

 read the number of degrees on the circle, and this will be the 

 angle or its complement. It frequently happens that the 

 micrometer has to be moved in or out to make the lines on 

 it accord with the second side; but as this motion is altogether 

 a parallel one, the accuracy of the measurement is not at all 

 affected. The simplicity of the mechanical arrangement can 

 now be very readily seen. The same advantage in using the 

 micrometer with every eye-piece belongs to the goniometer. 



