5872:]\. ° Light. BLY 
The difficulty of obtaining large specula for telescopes, together with the 
disadvantages attending the weight, the brittleness, and liability to oxidation, 
of the speculum metal generally used, has induced Mr. Nasmyth to turn his 
attention to the employment of silvered plate glass for telescopic purposes, as 
it possesses perfect truth of surface, is lighter than metal, is not liable 
to oxidation, and a greater quantity of light is reflected from it than from any 
metallic surface. To give a concave or convex form to a disc of plate glass, a 
certain pressure must be made to act equally over the surface. This equal 
pressure is obtained on Mr. Nasmyth’s plan by taking advantage of the weight 
of the atmosphere. A disk of silvered plate glass, 39 inches in diameter, and 
3-16ths of an inch in thickness, is fitted and cemented into a shallow cast-iron 
dish, turned true on its face so as to render the chamber behind the glass per- 
fetly air-tight; by means of a tube communicating with this chamber, 
any portion of air can be withdrawn or injected. To produce a concave 
mirror so slight a power is required, that, on applying the mouth to the tube 
and exhausting the chamber, the weight of the atmosphere, which amounts in 
this case to 3558 pounds, acting with equal pressure over a surface of 1186 
square inches, causes the glass to assume a concavity of nearly three-quarters 
of an inch, which, in a diameter of 39 inches, is far beyond what would ever be 
required for telescopic purposes. On re-admitting the air, the glass imme- 
diately recovers its plane surface, and on forcing in air with the power of the 
lungs, it assumes a degree of convexity nearly equal to its former concavity. 
The degree of concavity or convexity may be regulated to the greatest nicety, 
and it is proposed to render the degree of concavity constant by placing in the 
air-tight chamber a disk of iron turned to the required form, and allowing the 
pressure of the atmosphere to retain the glass in the form given to it by its 
close contaé with the iron disk. The curve naturally taken by the glass when 
under the pressure of the atmosphere, is believed by Mr. Nasmyth to be the 
catenary, inasmuch as its section would be the same as that of a line sus- 
pended from each end, and loaded equally throughout its#length. 
From America we hear of an ingenious application of photography as an 
aid to locksmiths. Several of the “leading railway lines in America have 
already become bonded carriers, and are ‘engaged in the transportation of 
imported goods from New York to the interior, under custom-house seal. The 
peculiar seal used for this purpose shows the practical value of photography to 
the industrial arts. The photographer for the Treasury Department is now 
engaged in preparing the seals for the new locks, to be used by that depart- 
ment in the transportation of merchandise in bond, and in such other cases 
where the protection they afford will be necessary. The lock itself is nothing 
more than an ordinary padlock, which is provided with an arrangement by 
which asmall piece of glass an inch square is passed over the key-hole and 
_ held in place by a small spring, which cannot be reached without breaking the 
glass itself. By no possible exercise of ingenuity can the lock be picked 
or opened without breaking this piece of glass. Here comes in the value of 
photography. A large sheet of glass, red on one side, is prepared in New 
York, by marking it off into squares of the proper size. On each square 
is marked a number in figures and irregular spots in red, the rest of the red 
surface being cut away with hydrofluoric acid. One of these sheets cannot be 
duplicated. The Government photographer receives them at Washington, 
and makes three photographs of them, which give perfe&t fac similes of the 
figures and spots on the glass, and then both glass and photographs are cut 
into small squares corresponding to each other and packed in boxes, each 
square of glass having with it three copies on paper. These are forwarded to 
the officers who will use them. The officer at New York, for instance, whose 
duty it is, locks the doors of the car containing bonded goods, places the glass 
square over the key-hole, and forwards the photograph of the same to the 
officer at Philadelphia or elsewhere whose duty it is to receive the goods. If 
on thearrival of the car the lock has been disturbed, the inspector is at once 
aware of it, and the company transporting is liable in bonds required pre- 
viously. This is an ingenious and praétical application of photography to the 
mechanical arts, and suggests numerous other applications of the art to the 
