SCIENTIFIC SEMMART. 
337 
"being very long from top to bottom, and very narrow from side to side. The 
effect given to these slides in the instrument, is remarkably good, the objects 
appearing to stand out more round and distinct, and with a larger share of 
naturalness than we obtain in the ordinary stereoscope. 
Neiu Lenses. — Mr. Ross constructed for the Abyssinian expedition a doublet 
lens; the diameters of the combinations were three inches and a half, and the 
equivalent focus four feet. This startling novelty played its allotted part 
perfectly. Another doublet of the same focal length was constructed by 
the same eminent optician for the Belgian government, the combinations of 
which monster lens are eight inches diameter ! Mr. Ross is now completing 
the largest portrait lens ever made in this country, for the Indian govern- 
ment. The diameters of the components are ten inches ! Another novel 
doublet lens has also been constructed by Mr. Rosa, for Mr. Stuart, the 
well-known photographer. This is for interiors and sea views. It is six 
inches focus, and wiU cover a 4 x 3 plate, sharp to the edges. 
PHYSICS. 
The Contraction of bodies during Solidification. — In the course of a dis- 
cussion which took place at the Chemical Society, some time since, conse- 
quent on the delivery of Mr. Chance’s lecture, Mr. David Forbes, F.R.S., 
stated that Mr. Chance, at his suggestion, compared the measurement of the 
modern frames used for making the moulds for casting certain blocks of 
basalt, with the blocks so prepared when cold, and it was ascertained that 
no difference of size existed. From this it was inferred that molten basalt 
neither expands nor contracts during solidification. In reference to this 
conclusion Mr. A. Tribe writes a letter to the Chemical News, of March 27. 
In this he states, that contraction may occur in various substances, and yet 
manifest itself not by any diminution of size, but by the formation of 
cavities in the interior. He advances certain instances in proof of his views. 
“ Melting Metal in a Handkerchief.^^ — This old and popular experiment is 
so often performed, both by lecturers and amateurs, that the following im- 
proved modification of it, suggested by Mr. Woodward, of the Birmingham 
Midland Institute, deserves notice : — Two or three pounds of fusible alloy 
are melted, and run into an evaporating dish ; when cold, a handkerchief is 
stretched over the smooth convex form thus obtained, and the mass may 
then be melted over a Bunsen’s burner in the course of a few minutes ; on 
piercing the handkerchief the melted metal runs out, and may be received 
in a mould. 
The Refraction- Equivalents of Substances * — Those who wish to read a short 
and intelligible paper on the results of the most recent inquiries on this 
subject, and to learn how eve?'g substance has a refraction com'pounded of the 
refractions of its constitueiits, should consult the reprint of Dr. J. H. Glad- 
stone’s lecture, in the Journal of the Royal Institution. The lecture was 
delivered on the 24th of April, and is already published in “proof.” 
Explosion in Paraffin Lamps. — We are glad to see that this point formed 
the subject of a paper which was read before the Chemical Society, at its 
meeting on the 4th of June, by Dr. B. H. Paul. The author pointed out 
