1871.] Light, in 



corresponding to those at the bases of the prisms. To each of these bases is 

 attached a bar, b, perpendicular to the base of the prisms. As all these bars 

 are slotted and run on a common centre, the prisms are brought into a circle. 

 This central pivot is attached to a dovetail piece of two or three inches in 

 length, placed on the under-side of the main plate of the spectroscope ; which 

 is slotted to allow it to pass through. On moving the central pivot, the whole 

 of the prisms are moved, each to a different amount in proportion to its distance 

 in the train from the first or fixed prism on which the light from the slit falls 

 after passing through the collimator, c. Thus, supposing the first prism of 

 the train opposite c, represented in the diagram, to be stationary, and the 

 second prism to have been moved through i° by this arrangement, then the 

 third prism will have moved through 2°, the fourth through 3 , the fifth through 

 4 , and the sixth through 5 . As these bars are at right angles to the bases of 

 the prisms, and all of them pass through a common centre, it is evident that 

 the bases of the prisms are at all times tangents to a common circle. Now 

 for the contrivance by which this arrangement is made automatic. A lever, l, 

 is attached to the corner of the triangular plate of the last prism. This lever 

 by its further end is attached to the support which carries the telescope through 

 which the spectrum is observed. Both the telescope and lever are driven by 

 the micrometer-screw, M. The action of the lever is so adjusted that when the 

 telescope is moved through any angle it causes the last prism to turn through 

 double that angle. The rays which issue from the centre of the last prism are 

 thus. made to fall perpendicularly upon the centre of the object-glass of the 

 telescope, t, and thus the ray of light travels parallel to the bases of the 

 several prisms, and ultimately along the optical axis of the telescope itself, 

 and thereby the whole field of the objedt-glass is filled with light. Thus the 

 apparatus is so arranged that on turning the micrometer-screw, so as to make 

 a line in the spectrum coincide with the cross wires in the eye-piece of the 

 telescope, the lever, l, attached to the telescope and prisms, sets the whole of 

 the prisms in motion, and adjusts them to the minimum angle of deviation for 

 that portion of the spectrum. 



The Paris correspondent of the " Engineer," in an interesting letter sent by 

 balloon post, speaking of the use of the electric light in war, says: — "War 

 certainly does something for science. The use of the electric light is common 

 to both sides in the present struggle, but the French have used it largely. The 

 apparatus set up on Montmartre is arranged by M. Bazin, and is electro- 

 magnetic. The central cylinder supports four series of double coils, covered 

 with copper wire, enveloped in silk ; the cylinder is rotated by a small steam 

 engine of three-horse power, making 400 revolutions per minute. The lamp 

 used is of the ordinary form, with the Foucault-Duboscq regulator. The 

 reflector is parabolic in form, and the whole is surrounded by a shield to hide 

 it from the enemy. This light, from its elevated position, commands the 

 whole of Paris and the plains around. A spectator on Montmartre sees 

 distinctly the details of the fagade of a building which stands 2600 metres off; 

 at 2900 metres a man may be seen standing at a window ; at 3000 metres a 

 mass of cavalry or infantry is distinguishable ; and at 4000 metres the dome of 

 the Invalides, with its bands of gold, is brilliant. A man cannot be seen on 

 the dome at that distance, but on walking towards the building all soon 

 becomes clear. On the ramparts, at 3800 metres from Montmartre, the light 

 is sufficient to read an ordinary newspaper. Thus, though the practical effect 

 of the lamp only extends about 300 metres from its position, the field is 

 illuminated to the extent of 700 metres, for the benefit of all placed between 

 the light and the object. Thus a sentinel on the ramparts can see about 3000 

 metres from the enceinte, and by this means strict watch is kept upon the plains 

 around the city at night, as far, in one direction, as 1000 metres beyond St. 

 Denis. M. Bazin is now occupied in applying his apparatus to the 

 purposes of night telegraphs, by the adoption of the system of flashes, and 

 with the aid of coloured lenses. A corvette, the Coligny, already possesses 

 such a signal apparatus, and thejsignals are distinctly visible at more than eight 

 miles distance." 



