NATURAL rillLOSOPHY. 157 



such accurate mathematical principles as will prevent the possibility 

 of error. The machinery is somewhat similar to that by which the 

 reflector of Lord Rosse's gigantic telescope was polished, with the 

 difference, that, the reflector being concave, and the Craig lenses 

 convex, the machinery will act reversely." 



The Observer states the following additional facts of an interesting 

 character, relative to the improvements made by the two scientific 

 gentlemen above named, the benefits of which will perhaps be even 

 more widely extended than those resulting from the single instrument 

 already perfected. " Not the least of the benefits which Mr. Craig- 

 has conferred upon astronomical science, is the practical demonstra- 

 tion of the fact, that achromatic telescopes of this vast size and extra- 

 ordinary range, may be constructed at a> comparatively small cost, 

 thus doing away with the necessity for the more expensive and elabo- 

 rate arrangements required for the great reflecting telescopes. The 

 simple and effective mechanism devised by Mr. Gravatt is another 

 illustration of the advance we have made in the mechanical arts, and 

 fully justifies the soundness of the judgment evinced by Mr. Craig 

 in his selection of an engineer." 



TRAVELLER'S CAMERA. 



MR. Fox TALBOT publishes the following description of a portable 

 photographic apparatus arranged by him, and called " The Traveller's 

 Camera." 



He says : I first mount the camera itself upon a board of its own 

 breadth, but two or three inches longer than it. I then make a kind 

 of table, or support, beneath the surface of which are sunk or con- 

 cealed three troughs, which are retained in fixed positions. One of 

 these is intended to hold a solution of nitrate of silver ; the second, 

 either a solution of gallic acid, or sulphate of iron ; and the third, 

 water. The usual paper holder is dispensed with, but instead of it 

 there is a simple frame, to which a sheet of paper or a pane of glass 

 can be attached from behind, and taken away again, while the frame 

 remains in the camera. The upper part of the frame carries a long 

 handle, passing through the lid of the camera, which may either stand 

 upright, or, if it be jointed, it can be folded down on the camera. 

 When the camera is placed on its table, or support, it can move upon 

 it in one direction only, backward and forward, being confined to 

 that motion by two parallel strips of wood, upon which are placed 

 certain marks corresponding to a mark upon the camera, indicating 

 that when either of these marks are brought into union, then the 

 paper holding frame of the camera is in a vertical line over the centre 

 of one of the troughs.. Now, when the photographer sets out of a 

 morning upon his excursion, he carries with him two boxes, one con- 

 taining the plates of glass (or the sheets of iodized paper) he intends 

 to use, which of course may be freely exposed to light, not being in a 

 sensitive state ; and the other box to hold the pictures which he 

 expects to make. AVhen arrived at the scene of action, the modus 



