SCIENTIFIC SUMMARY. 
443 
seed to such as would be mounted in a small locket. The one-inch lens is 
that preferred for general use, but a lower power is also provided. Both 
of these object-glasses have their photographic and optical foci coincident ; 
no allowance need therefore be made in the adjustment of the sensi- 
tive plate ; and the fact that Mr. Eden was for some years a pupil of 
Mr. Andrew Pritchard is so far a guarantee for the perfection of the 
optical part of the microscopic camera. 
ROFESSOR W. A. MILLER, of King’s College, London, has 
published in the Proceedings of the Royal Society an abstract of 
an important investigation “ On the Photographic Transparency of various 
Bodies, and on the Effects of Metallic and other Spectra obtained by means 
of the Electric Spark.” The first portion of his paper refers to the absorbent 
action of various media upon the chemical rays of the electric spark, the 
spark or light being obtained between two metallic wires (generally of 
fine silver) connected with the ends of the secondary wire of a strong 
induction coil. The light was first passed through a narrow vertical slit 
in a piece of metal, then through the substance to be examined, which, if 
a liquid, was contained between two thin plates of polished rock-crystal, 
then through a quartz prism placed at a suitable distance, immediately 
upon this, through a lens of rock-crystal; and at a suitable distance 
the spectrum was received upon a sensitive collodion surface in a camera. 
His experiments with gaseous substances were made by interposing in 
the track of the ray, between the vertical slit and quartz prism, a brass 
tube, two feet long, closed at each end, air-tight by means of a plate of 
quartz, and each gas or vapour was introduced separately into the tube. 
His general results were: — 1st. That colourless bodies which are 
equally transparent to the visible rays vary greatly in permeability to 
the chemical rays ; 2nd. Bodies which are photographically transparent 
in the solid state preserve their transparency in the liquid and gaseous 
states; and 3rd. Colourless transparent solids which exert a considerable 
photographic absorption, generally preserve that power when converted 
into liquids or gases, and it makes no difference whether they are liquefied 
by means of heat or by being dissolved in water. He employs the term 
“ diactinic ” to express the power which bodies possess of transmitting 
the actinic or chemical rays. 
The experiments upon gaseous bodies gave important results, showing, 
however, but little coincidence with those of Tyndall on the absorbent 
power of gases for radiant heat. Amongst the colourless gases, atmo- 
spheric air, oxygen, hydrogen, nitrogen, carbonic oxide, and carbonic acid, 
exert no absorbent power ; olefiant gas, nitrous oxide, cyanogen, and 
hydrochloric acid slightly absorb the chemical rays ; whilst with coal 
gas the more refrangible half of the chemical spectrum was entirely cut 
off. Then follow, with increasing power, sulphurous acid, bisulphide of 
carbon, and terehloride of phosphorus. Atmospheric air saturated with 
PHYSICS. 
