158 Intelligence and Miscellaneous Articles. 



the object ; in short, the image will be deformed, contractions and 

 dilatations -will be seen, showing a decrease or an increase of the 

 radius of curvature of the corresponding element of the mirror. The 

 method becomes more correct and more conclusive if, instead of 

 allowing the edge to have an indeterminate position in space, we fix 

 on the eyepiece of a small telescope, magnifying but little, and pro- 

 vided with a narrow diaphragm comparable with the pupil. 



It is very rare that an image observed in such way will show itself 

 free from any distortion, and such as a perfectly spherical mirror 

 would give it ; the edges, instead of remaining rectilinear, are bent 

 most frequently in the shape of an hyperbola, and turn their con- 

 cavity towards the interior of the field of the telescope ; in presenting 

 themselves under that shape they indicate that in every meridional 

 section of the mirror the curvature goes on gradually diminishing 

 from the centi-e towards the edge, which tends to correct to a cer- 

 tain degree the spherical aberration. 



Lastly, it remains now to describe a process which allows of the 

 examination of concave surfaces, so as to recognize directly, by an 

 effect of relievo, the elevated and depressed parts which affect the 

 figure. 



A thin piece of metal, pierced with a small hole of -pjth of a 

 millimetre (0"00328 inch, or about ^j^th of an inch), and enlight- 

 ened by an artificial light, is placed within the centre of curvature ; 

 through that little hole the diverging pencil of rays falls on the sur- 

 face which is being examined, and comes back converging to form an 

 image which is situated a little beyond the centre. 



By placing the eye in the cone of rays which diverge anew, and 

 bringing it towards the image, we at last receive the whole of the 

 pencil of rays, which passes freely through the pupil ; and at the same 

 time the surface of the mirror will be seen entirely illuminated. 

 Now if an opake and rectilinear edge be brought near the image of 

 the hole and made to infringe on it by degrees, the mirror will 

 also lose by degrees its brilliancy ; and when all the light is about 

 to disappear, the whole of the irregularities of the surface of the 

 mirror will be plainly seen. 



In fact, supi)osing the surface to be correctly spherical, the image 

 is perfectly well defined, and when allowed to emerge a little from 

 the opake screen which is brought forward to intercept it, the rays 

 which are not stopped off come from the whole surface of the mirror, 

 which presents to the last a uniform brilliancy ; if, on the contrary, 

 this surface shows here and there some parts which are situated above 

 or below the spherical level, the elements which do not present 

 themselves under the proper incidence cause certain rays to deviate, 

 instead of converging with their fellow rays (congenh-es), towards 

 the different parts of the image, and to be dispersed in all directions. 



It results from this, that every point of the image, and in particular 

 each of those which emerge from the opake screen, receives rays 

 which do not belong to it, and does not receive those which do be- 

 long to it. 



The missing rays will leave in their corresponding places on the 

 surface of the min'or a defiiciency of light, the accumulated rays will 



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