1887. | MICROSCOPICAL JOURNAL. 13 
approximate or comparative results. A method that is said to give absolute 
numerical results very expeditiously has lately been proposed by Mr. Gordon 
Thompson. His method is as follows :— 
A fine mark is made on an ordinary glass slide with a writing diamond. 
A large and very thin cover-glass is cut in half and the pieces cemented to 
the slip on either side of the mark, leaving a space of about one-eighth of an 
inch between their edges. A very thin cover-glass is then placed upon them, 
thus forming a cover to a rectangular cell, in which the liquid to be exam- 
ined is placed. 
The fine mark is now viewed with the highest power available, and when 
the focus is sharp the position of the fine adjustment is noted. | The slip is 
then removed, another fluid substituted, using the same cover-glass, and the 
operation repeated. The change in the focal adjustment required is a meas- 
ure of the difference in the refractive indices of the two fluids. In this way 
the relative refractive powers of two or more media can be immediately 
determined. 
To measure the absolute index of refraction of a fluid it is necessary to 
obtain a numerical value of the graduations of the fine-adjustment. To do 
this, choose two liquids of known refractive power, one of low index, as water, 
and the other of high, as oil of cassia. Focus through them both successively, 
and note the alteration of focus required. The indices of the fluids being 
known, a value for each division of the fine-adjustment is easily obtained by 
Z 
calculation. The method is based upon the formula of refraction, v= +~, 
in which v is the position of the geometrical focus of direct rays passing 
through a plate whose thickness is ¢. 
It is not, however, a new method, nor can it be depended upon for abso- 
lutely accurate results, although for all practical purposes of the microscopist 
it will serve perfectly well. Far better determinations can be made with Prof. 
Abbe’s refractometer.—H. 
O 
ABSORPTION OF COLORING MATTERS BY THE LIVING PROTOPLASM OF VEG- 
ETABLE CELLS.— The following extract from a recent work on vegetable 
physiology expresses in a few words the generally-accepted view relative to 
the action of dyes on vegetable cells :—‘ It is impossible to stain living pro- 
toplasm; it is when protoplasm is dead that coloring matters can penetrate 
into it.’ The first part of this statement requires an important qualification. 
Passing by certain well-known facts relative to the transient tinging of ani- 
mal protoplasm in certain cases, noted by Heidenham, Brandt, and Dreser, 
attention is asked to the recent discovery by Pfeffer in Unters aus dem botan. 
Inst. Tiibigen Bd., ii, iv. | Professor Pfeffer shows that when living vegeta- 
ble cells are placed in very dlute solutions of certain coal-tar coloring mat- 
ters, the protoplasm becomes distinctly colored, and remains so for a time. 
The best results are obtained by placing roots with attached root-hairs in half 
a liter of pure water, to which is added one ten-thousandth of one per cent. 
of almost any of the so-called methyl colors, such as ‘ methyl-green,’ ‘ methyl- 
violet,’ ‘ methyl-orange,’ and such colors as safranin, Bismarck-brown, and the 
like. Nigrosin and eosin, and two or three others, are not well adapted to the 
purpose. After a short time, especially if the specimen is shaken in the solu- 
tion, the protoplasm will be found distinctly tinged. Buta few colors, nota- 
bly methylene-blue, do not color the protoplasm at all, but impart to the cell- 
sap an intense color. In this case the dye has passed through the protoplasm, 
without tinging it, into the cell-sap which receives it.—G. Z.G., Am. Jour. 
Scz., vol. 29, p. 486, 1886. 
