Intelligence arid Miscellaneous Articles. 159 



produce tin increase of intensity in others ; and from the contrast of 

 these different degrees of brilliancy, and on account of an effect of 

 clair-obscur, an enormously exaggerated manifestation of the inequal- 

 ities which really exist on the surface submitted to this sort of test 

 will result. 



As the observation can only be made by means of a single eye, it 

 happens that, under a mental influence independent of the will of the 

 observer, the valleys and hills seem to overlie each other ; but with a 

 little attention, what is a mere illusion will soon be distinguished 

 from reality by considering that the true rising parts are neces- 

 sarily contained between two opposite declivities, one of which being 

 turned towards the screen, which hides the image, appears dark, 

 whilst the other presents an increase of brilliancy. In other words, 

 the true figure of the mirror is analogous to that which would corre- 

 spond to the apparent relievo interpreted on that hypothesis, in which 

 an oblique light would come from the side opposite to the screen. 



This process offers particularly the advantage of being at the same 

 time expeditious and direct ; that is to saj^ a surface can be judged 

 of in a few seconds, and when it is defective, the faults of it are 

 known immediately with great precision. 



Generally, all that has any influence on the direction of the rays, 

 or anything which prevents them from converging simultaneously 

 towards a point of common convergency, becomes visible and is de- 

 tected. By means of this kind of observations, among several causes 

 which affect the result, that one which exercises a predominant influ- 

 ence is easily distinguished from the rest. Thus the permanent 

 irregularities of a surface will not be mistaken for the varying altera- 

 tion due to the flexure of the mass of glass ; and we can perceive 

 the inequalities in the density of the ambient air, which passes 

 whirling before the mirror, or which shows itself like stratified layers 

 in the interior of the tube of the telescope. By taking the necessary 

 precautions, all causes capable of producing an accidental disturb- 

 ance are successively avoided, and the intrinsic faults or defects of 

 the surface are the only ones which subsist. 



But, guided by the observation itself which causes them to be 

 discovered, we succeed, as I have demonstrated before, in correct- 

 ing locally these defects ; and, lastly, they can be reduced to such 

 proportions as to exercise no sensible influence on the quality of the 

 images. 



The mechanical processes by which the working of glass surfaces 

 is usually effected seem to lose something of their efficacy as soon 

 as they are applied to pieces of unusual dimensions ; the results 

 which they then furnish are but an approximation, a sort of trial 

 which is far from being satisfactory ; but where mechanism becomes 

 powerless, the hand of man can do something more : assisted by the 

 resources placed at his disposal by optical science, and guided by a 

 system of observations the efficacy of which increases with that of the 

 instrument whicli is to be constructed, the human hand is enabled to 

 proceed with the work, and to carry it out to the greatest degree of 

 precision. 



If it be required to verify by optical means any one of the ellip- 



