NATURAL rinr.osoi'HY. 159 



the exact area covered by the flash of the mirror. It is scarcely possible to 

 describe the instruments that were submitted to the Association, without 

 drawings. They consisted of a tube of wood fifteen inches long, and with 

 an eye-hole at one end; a mirror turned on an axis at right angles to the 

 tube ; and, in front of the mirror, a slip was cut away from the side of the 

 tube, and the lens was inserted athwart the cut-out part. Part of the lens 

 projected within the tube, and part outside of it, and in front of the mirror. 

 The screen was placed at the further end of the cut-out part, and an envelop 

 protected the whole from injury. A slide in front of the lens regulated the 

 amount of light thrown on it, and toned the image to the required degree of 

 brightness. The addition of a telescope was not found practically useful. 

 Neither was that of a second mirror, for double reflection, to meet the diffi- 

 culty of sending signals when the sun was behind the back of the signaller. 

 It is not difficult to signal within twelve degrees of the point opposite to the 

 sun, and it is possible to do so within seven degrees. The looking-glass 

 should be of the very best plate-glass, and it ought to have its sides truly 

 parallel, else there will be a confusion of images and an irregularity in the 

 flash. Letters are conveyed by treble groups of flashes, each of which 

 groups consists of one, two, or three flashes, as the case may be. 



TELESCOPIC MIRRORS. 



Foucault has communicated to the Academy of Sciences a memoir on 

 the substitution of silvered glass for metallic alloys in the construction of 

 reflecting telescopes, and on the possibility of producing surfaces of revolu- 

 tion which reflect parallel rays to a single focus. He remarks, in the first 

 place, that the spherical sin-face itself is difficult to obtain with absolute 

 accuracy. When a luminous point is placed in the centre of curvature of a 

 concave mirror, the image of this point is usually surrounded by an aureole, 

 the appearance of which indicates defects in the surface. The author reme- 

 dies these defects by retouching the mirror in different parts, until the image 

 is free from faults. The spherical mirror is then converted into an ellipsoidal, 

 and finally into a paraboloidal mirror, by successive processes of approxima- 

 tion. A luminous point, placed at first in the centre of curvature, is made 

 gradually to approach the principal focus ; the image, of course, recedes in 

 the opposite direction. By means of an appropriate polisher, the figure of 

 the mirror is corrected for each successive portion of the luminous point, until 

 finally the aberration becomes invisible for parallel rays. With a telescope 

 constructed in this manner, with a mirror thirty -three centimetres in diame- 

 ter, and having a focal length of 2m-25 the author succeeded in resolving the 

 blue star of y Andromedas into two distinct points. This result had hitherto 

 been obtained only by Struve with the large Pulkowa instrument. Comptes 

 Hendus, xlvii. 205. 



ON THE DANGER ATTENDING THE USE OF RED AKD GREEN LIGHTS 



AT SEA. 



The following is an abstract of a most valuable practical paper submitted 

 to the British Association, 1858, by Prof. G. Wilson. It commenced by stating 

 the Admiralty regulations, that " 1. All sea-going vessels, when under way, 

 or being towed, shall, between sunset and sunrise, exhibit a green light on the 

 starboard side, and a red light upon the portside of the vessel. 2. The colored 



