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POPULAR SCIENCE REVIEW. 
of a rigid body, as in Eden’s; and employing a “mount” for the 
lens, stage, mirror, &c., that allows of the following adjustment : namely, 
the lens being brought nearer to or further from the stage, according 
as it may be of long or short focus ; fine adjustment for the object, 
with the means of registering the difference between the chemical and 
visual foci for each lens, or object , according to the medium in which it may 
be mounted ; gymbal adjustments on the stage, in case the object should 
lie obliquely instead of parallel to the objective ; a secondary stage for the 
condenser and polarizer, and a plane mirror or achromatic prism with 
universal motions for the illumination of the object, as shown in the 
annexed diagram. With high powers, the allowance for the difference of 
the two foci is best effected by means of the micrometer head adjustment ; 
with low powers, the object-glasses themselves should be corrected. By 
reversing the arrangement of parts, micro-photographs may be taken 
by the same instrument. 
In connection wth the question of a National Portrait Gallery, it has 
been suggested that M. Joubert’s “ burnt-in ” photographs on glass might 
be made available ; or positive transparencies on glass, sealed up between 
two plates of glass, should be employed, as being the most indestructible 
form of photograph, the negatives from which they would be produced 
being likewise sealed up between glass and kept in a place of special 
safety. Photographers are not yet agreed as to the best material for the 
substratum for the collodion film for dry plates. Mr. Glover advocates a 
weak solution of india-rubber in benzole, while Mr. Taylor declares for a 
very dilute solution of albumen, which is the medium recommended by 
Mr. Barnes in one of the earliest works on the dry processes. 
Professor Emerson Reynolds calls attention to the value of the arnmo- 
niacal dichloride of copper for the precipitation of silver salts from waste 
solution. 
Several physicists are again giving attention to the construction of actino- 
meters that can be made practically available ; the most promising of the 
arrangements being that suggested by Professor Roscoe of Manchester. 
The British Journal of Photography , one of the oldest of the periodicals 
devoted to the science, which has hitherto appeared fortnightly, announces 
that in future it will be published weekly, and that its publishing depart- 
ment will be removed from Liverpool to London. 
PHYSICS. 
Defective Spectroscopes. — In a communication to the American Journal 
of Sciences (XXXIV.), Mr. Rutherford describes the spectroscopes now 
employed by Donati, Secchi, and Professor Airey, in their observations 
of stellar spectra, and points out objections to each form. Donati uses a 
large burning-glass, fifteen inches aperture, with a focal distance of sixty- 
two inches, not achromatised, and mounted equatorially. The cone of light 
passes through a fine slit before reaching the focal point ; after crossing 
at the focus, it traverses a cylindrical lens ; thence, after being rendered 
parallel by an achromatic lens, it falls upon a flint-glass prism of 61 °; 
emerging from this, it is received by a small achromatic objective, and is 
