368 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
holder ; it also contains an account of the method of examination of a 
crystal by immersing it in a liquid with refractive index the mean of 
the refractive indices of the crystal. 
The Microscope employed (made by Fuess) is of the petrological 
type, with circular rotating stage provided with rectangular movements. 
The nicols are made to rotate simultaneously, as in the Allan-Dick 
Microscope made by Swift. 
The stage goniometer is used with the Microscope in the horizontal 
position. The base-plate of the goniometer is clamped upon the now 
vertical stage. It carries a horizontal projecting piece, terminating in 
the supporting cone for the goniometer circle. The adjusting movements 
of the goniometer are simpler and lie closer together than in the ordinary 
form of goniometer. The glass cell containing the immersion liquid is 
supported by a stand with adjustable arm placed to the left of the 
Microscope. 
The determination of the true optic axial angle 2 Y can be made at 
once with the instrument if the crystal is immersed in a liquid with 
refractive index equal to the intermediate refractive index /3 of the 
crystal. 
The advantage of the method'eonsists in the fact that it is unnecessary 
to prepare sections of the crystal, for if the immersion liquid is properly 
chosen, the crystal will lose its outline and disappear in the liquid. 
For observation by this method, crystals should be chosen which are 
equally thick in two perpendicular directions in the plane of the optic 
axes, and not too thick to prevent the interference figures being 
observed. 
Prof. Klein describes the method of determining with the apparatus 
the extinction angles upon the various faces of a zone, and shows how 
the method introduces a great simplification in the study of triclinic 
crystals, since the position of the optic axes can be at once determined. 
Mr. Tutton shows what use he made of the method of observa- 
tion in the course of his investigation of the sulphates of potassium, 
rubidium, and caesium.* 
Indication of Magnification in Micrographic Drawings.! — Dr. D. 
Carazzi remarks that there are still naturalists who preserve the bad 
habit of not giving the indications of the magnification in the micro- 
graphic drawings which illustrate their memoirs, although nearly all are 
convinced of the necessity of so important an indication. The usual 
plan is to give with the drawing the number of the eye-piece and objec- 
tive, the name of the maker, and the length of body-tube. Dr. De Yescovi J 
has pointed out that these three elements are not sufficient, and that 
account must also be taken of the real size of the object, and of that of 
the drawing obtained with the camera lucida. But as the operations 
involved in this method are long and troublesome, the former plan is 
still usually adopted. The author shows, however, how impractical and 
useless the method is, owing to there being no rational system of notation 
amongst Microscope makers. For this reason the name of the maker 
has to be given, and the reader has to refer to a number of makers’ 
catalogues in order to understand the indication. 
* Journ. Chem. Soc., 1894, p. 628. f Zool. Anzeig., xviii. (1895) pp. 162-4. 
X Op. cit., x. (1887) p. 197. 
