18 



ON THE C O N S T E IT C T I O X A X D USE OF 



principal i'oral length. That is, if a 15| inch mirror of 150 inch(>s focal length 

 were spherical, and were nsed to c-onverge parallel rays, those from its edge wonld 

 reach a focns j;^„ of an incli nearer the mirror than those from its central parts. 

 If now the conv(>rse experiment be tried, and a mirror of tlie same size and focal 

 length which can converge parallel rays, falling on all its parts, to one focns, be 

 examined at the centre of curvatnre, it gives there an amount of longitudinal 

 aberration ^y,, of an inch, equal to twice the preceding. This latter, then, is the 

 condition at tli(> centre of cur^anire, to whicli sucli mirnir must l)e brought in order 

 to converge parallel rays with exactness, lu addition, strict watch must be kept 

 upon the zones intermediate between the centre and edge, both by measurement 

 with diaphragms of their aberration, and l)etter yet, by observation of the regu- 

 larity of the curve of that apparent solid. Fig. l(i, s(-en by tlu' second t(>st. 



This modification of the first test is literally a method of paralxdi/.ing l)y measure, 

 and is capable of great precision when the eye learns to estinnite where the exact 

 focus of a zone is. The little irregidarities fovuid roiuid the edges of the holes 

 through the tin screen, Fig. 8, arc in this respect of material assistance. They 

 show, too, the increased optical cu' penetrating power that is gained l)y increase of 

 aperture. ISlinute pecidiarities, not visible nn(h'r very higli powers witli a 10 inch 

 diaphragm, l)ecome immediately perceptible even with less magnifying wlien tlie 

 Avhole aperture is used, ])ro\ided the mirror is s])lierical. 



In the use of the second test precautions have to be taken, as may be inferred 

 from page 15, to set the opaque screen exactly in the jjroper position. The best 

 method for ascertaining its location is, h.aviug received the image into the eye, 

 placed purposely too near the mirror, to cause the scn^en to move across the cone 

 of rays from tlu- right towards the left side. A jet black shadow begins to advance 



at the same time, and in the same directitni 

 across the mirror. If the eye is tlien moNcd 

 from the mirror sutficiently, tliis black sliadow 

 can be made to originate by tiie same motion 

 of the screen as before, from the left or oppo- 

 site side of the mirror. INlidway between these 

 extremes there is a ])oint where the advance is 

 from neither side. This is the true position 

 for the screen when it is desired to see the im- 

 p(n-fections of the surfac(> in highly exaggerat- 

 (h1 relief as in Fig. 20, whicli re^jresents the 



Kg. 20. 



ap])earance of Fig. 12.^ 



The interpretation of the lights and shadows 

 upon the face of a mirror in this test is always 

 easy, and the observer is not likely to mistake 

 an elevation for a depression, if lie bears in mind the fact that the surface under 



Adjusting the Opaque Screen. 



' Tn order to examine Fi<r. 20, the book should be held with the left side of the page toward a 

 window or lamp. The eye should also be at least two feet distant. The centre will then be seen to 

 protrude, and the surface present the apparent section engraved below it. 



