Intelligence and Miscellaneous Articles. 157 



For 82°-5 (that is, on the temperature being raised from 17°'5 



The expansion in the direction of the axis of a is -001771 



6 is -001794 



Therefore cubical expansion for 82°-5 is . . . -005379 

 — Archiv d. Pharm. vol. cxlviii. p. 19. 



DESCRIPTION OF VARIOUS PROCESSES MADE USE OP FOR FINDING 

 OUT THE CONFIGURATION OF OPTICAL SURFACES. BY M. 

 LEON FOUCAULT. 



One of the principal advantages of the reflecting telescope con- 

 sists in having, instead of an object-glass, a mirror which acts by- 

 means of a single surface ; which surface being concave, lends itself 

 with remarkable facility to the application of the processes of exami- 

 nation which permit the appreciation of its figure even to the least 

 details. 



I conjointly make use of three different processes. The first con- 

 sists in placing very near to the centre of curvature a very small 

 object, such as the point of a pin, in order to obtain an image of the 

 same size as, and very near to the object itself; this image can then 

 be observed by means of a microscope and compared directly with 

 the object. We are thus enabled to judge with certainty, by means 

 of the amount of definition, of the effect which the mirror mounted 

 as a telescope will give. 



If the image is imperfect, it is better then, with the view of ascer- 

 taining the faults of figure, to take a luminous point and observe the 

 appearance of the pencil of light within and without the focus. It 

 will be seen to decompose itself in partial images, the discussion of 

 which gives at once some information respecting the configuration 

 of the surface of the mirror. 



But the two processes I am about to point out are still preferable, 

 and give this information with greater precision. 



An object having parallel sides, such as the edge of a thin piece 

 of steel, 1 millimetre (0-03937 inch) in thickness, is placed in 

 the neighbourhood of the centre of curvature ; and in order that 

 the object may project itself in outline, it is enlightened from the 

 side opposite to the mirror, and the image which is formed by it, 

 being on a luminous ground, becomes very apparent, and presents 

 in its aspect some peculiarities which depend upon the figure of the 

 mirror. This image, being observed with the naked eye at the di- 

 stance of distinct vision, is seen in each of its parts, but by means of 

 rays passing through the pupil after having been reflected from a 

 very small part of the surface of the mirror ; so that in these circum- 

 stances the mirror only acts but partially, that is, by more or less 

 restricted portions of its surface, which contribute severally to the 

 formation of different parts of tiie image. Thus if the curvature be 

 not uniform, the different i.arts of the image will not form them- 

 selves in Uie same plane, and the angles sul)ten(lcd at the eye by the 

 different parts will not be proportional to the corresponding part of 



