ZOOLOGY AND BOTANY, MICltOSCOPY, ETC. 
739 
the perfect combustion, tbe flame is perfectly white, smokeless, and 
devoid of odour. 
Magnifier for Reading Small Print.* — This^magnifier, which is now 
being offered for sale in the shops of Parisian opticians, consists, not of 
an ordinary lens, but of a thin-walled glass-tube filled with water and 
closed at both ends. This tube, which is in length about that of the 
lines of a book, is held at both ends in a piece of bent wire which is 
provided in the middle with a handle so that the instrument can be 
rolled over the lines of the print. 
The New Photometry. f — Prof. Crova is the inventor of new and 
improved methods of photometric work. The use of the electric light 
has been the means of exposing the many imperfections of the old photo- 
metric processes. The standard sources of light, as candles, Carcel 
lamp, Hefner-Alteneck lamp, &c., have all great faults, and the various 
photometric apparatus are so imperfect that the simple law of the 
square of the distances in practice becomes a source of error. 
Prof. Crova finds that the Carcel lamp is the most reliable of all known 
standard sources of light, if the instructions of Dumas and Regnault for 
its use are rigorously followed. When, however, the Carcel lamp is 
used for measuring the intensity of an arc or incandescent light, it loses 
a great part of its value owing to the fact that the illuminated surfaces 
of the screen of the photometer have no longer the same colour. In the 
laboratory this difficulty might be overcome by isolating with the 
spectrophotometer the rays of 582 wave-lengths, and from the ratio of 
their respective intensities determining the total intensities of the two 
sources of light. Such a method requires expensive instruments and 
special knowledge. 
Prof. Crova therefore recommends a simple means for obviating the 
difficulty, which consists in observing the screen through a glass vessel 
with parallel walls filled with a solution of nickel and ferric chlorides. 
The proportion of the two salts is so chosen that the solution only 
transmits rays which have a wave-length of 582. 
The whiteness of the different sources of light, Crova defines as 
follows : — It is the ratio of the intensities of the rays whose wave-lengths 
are about 582 | u. and 650 /x. The intensity for the latter wave-length is 
obtained by observing the screen through a glass coloured red with 
cuprous oxide. Experiment has shown that this ratio is unity for sources 
of light of the same colour as the Carcel lamp, that it varies from 1 • 05- 
1 • 23 with the electric incandescent light, and from 1*5 to 1*7 with the 
arc-light. 
In photometric work the great differences of intensity of the sources 
of light offer a serious difficulty, since for direct working, a room 
20-30 m. long would be necessary. This difficulty is overcome by 
placing the intense source of light in a small side room, separated from 
the photometric room by a wall in which is an opening covered with a 
ground glass. This glass is covered with a screen in which is a hole 
of exactly a square centimetre, and the source of flight is exactly a metre 
from the glass. The square centimetre of the ground glass gives in the 
photometric room a light whose intensity is a certain fraction easily 
* Central-Ztg. f. Optik u. Mechanik, xv. (1891) p. 189. f Tom. cit., pp. 194-5. ' 
