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POPULAR SCIENCE REVIEW. 
flows through the floating valve, which again closes as the mercury rises in 
the upper vessel and forces through the platinum valve, and upper column 
of mercury, the air which has entered from the receiver. It is possible to 
obtain a very good vacuum in a larger receiver by the aid of this instru- 
ment. 
How Pressure influences Combustion. — This has been very well shown by 
M. Cailletet, an abstract of whose recent paper on the subject appears in 
“ Silliman’s American Journal,” May 1875. M. Cailletet has studied the 
effect of a pressure of 30 to 35 atmospheres on the luminous, calorific, and ’ 
chemical rays, emitted by a burning body. The air was compressed by 
pumps in which the pistons were fixed and the cylinders moveable, a layer 
of water or glycerine at the same time cooling the gases so as to protect 
the packing from the heat and preventing leakage. The reservoir consisted 
of a hollow cylinder with four apertures ; the first admitted the gas, the 
second allowed it to escape, the third admitted the manometer-tube, and the 
fourth was closed by a thick glass plate to allow of observation of the in- 
terior. The latter had a diameter of 10 cms. and a capacity of about 4 litres. 
Placing a lamp in this space, the brightness increased with the pressure of 
the air. The base of the flame, which under the ordinary pressure is blue 
and transparent, became white and very bright j but soon the appearance 
changed and thick clouds of smoke circulated through the apparatus and 
escaped by the stopcock closing the outlet. The flame seen through this 
smoke is reddish and the wick is found to be charred and much soot de- 
posited, doubtless owing to the dissociation of the gases containing carbon. 
Spectrum-photographs. — “ The Academy ” of April 24, states that in 
the last number of “ Poggendorff” H. W. Vogel describes a simple appa- 
ratus for photographing the spectrum of the sun, or other spectra. He 
removes the lens from an ordinary photographic camera, and replaces it by 
a pocket spectroscope, fitted into the aperture by means of a blackened cork. 
The sun’s rays are allowed to fall on the apparatus parallel to the axis of the 
spectroscope. The lines, though not very sharp, are sufficiently so for many 
purposes, e.g., for the comparison of absorption spectra. 
Defects of the Human Eye as regards Achromatism . — A paper has been 
read on this subject before the Physical Society of London (April 10, 1875), 
by Professor H. M‘Leod. He said that the eye has been thought to be 
achromatic because it is so practically $ but it is easy to offer abundant evi- 
dence of the defects of the organ in this respect. For instance, to short- 
sighted persons the moon appears to have a blue fringe. In using the spec- 
troscope the red and blue ends of the spectrum cannot be seen with equal 
distinctness without adjusting the focussing glass. A black patch of paper 
on a blue ground appears to have a fringed edge if viewed from even a 
short distance ; while a black patch on a red ground, when observed under 
similar conditions, has a perfectly distinct margin. Professor M‘Leod then 
explained that the overlapping of images in the eye produces the mental 
impression that there is no want of achromatism. It is interesting to note 
that Wollaston considered that the coloured bands of the spectrum were 
really divided by the black (Fraunhofer) lines ; and his statement that the 
red end of the spectrum does not appear to have a boundary line 11 because 
the eye is not competent to converge the red rays properly,” shows that he 
