156 Chronicles of Science. | Jan. 
2501. It will be a triple achromatic, and the glasses will probably 
require months for their completion, during the whole of which time 
the grinding and polishing machinery will have to be moving under 
the personal superintendence of one of the first practical opticians in 
England. The experiment is necessarily a costly one, and photo- 
graphers are naturally anxious to see if the result compensates for the 
enormous additional expense. The Melbourne Government deserve 
the thanks of all photographers for the spirit of enterprise they have 
shown in the matter. 
From time to time rock crystal lenses have been recommended on 
account of their superior transparency to the chemical rays of light. 
Mr. Grubb has put this theory to the test of experience, and finds that 
the difference is not so great as has been imagined; for instance, a 
compound lens of the ordinary make transmits 87 for every 100 rays 
which the rock crystal allows to pass. It is therefore only 13 per 
cent. worse, whilst in flatness of field and achromatism, the glass lens 
is much superior. 
M. Gaudin suggests that lenses should be made from fused rock 
crystal. The manufacture of these is simply a question of expense, 
and they might possibly be achromatized by the employment of other 
suitably transparent minerals. 
A new fixing agent, sulpho-cyanide of ammonium, is likely ere 
long to supersede hyposulphite of soda. The advantages claimed are, 
permanence of the print, and great facility in the washing operations ; 
but, on the other hand, the expense is likely to be an objection. A 
little time ago, the new agent cost 4s. an ounce; there are rumours 
that it can now be procured in Paris for 1s. 13d. per Ib., although we 
have been quite unable to obtain any at this price, and Mr. Spence, 
the manufacturing chemist of Manchester, has just erected large ap- 
paratus, by means of which he hopes to supply the sulpho-cyanide at 
even a less price. We may therefore reasonably anticipate that sulphur 
toning, yellow whites, and fading positives, will soon have gone the 
way of the Dodo and Megatherium. 
Celestial photography is making great strides on the other side of 
the Atlantic. Dr. Henry Draper has just completed a large reflecting 
telescope, 16 inches in aperture, and 13 feet focus, which he intends 
to devote to this branch of science. ‘he mirror is of glass, covered at 
Sir John Herschel’s suggestion, with a film of precipitated silver. It 
is sustained in a walnut tube, hooked with brass, and specially mounted 
to avoid tremor. When in use the instrument is allowed to be at rest, 
clockwork being used only to drive the sensitive plate. By this means, 
only 1 oz. instead of half a ton, is moved. A photographic laboratory 
is attached to the observatory, and the apparatus is arranged to take 
photographs of the moon as large as 3 feet in diameter, being on a scale 
of less than 50 miles to the inch. From the reputation which Dr. 
Draper has already earned as a photographer and physical philo- 
sopher, we are justified in expecting that celestial photography will 
advance rapidly in his hands. 
