18 



ON THE CONSTRUCTION A X P USE OF 



principal focal length. That is, if a 151 i no h mirror of 150 inches focal length 

 were spherical, and were used to converge parallel rays, those from its edge would 

 reach a focus I Tr of an inch nearer the mirror than those from its central parts. 

 If now the converse experiment be tried, and a mirror of the same size and focal 

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

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

 aberration ^"o f an " 1cn ' equal to twice the preceding. This latter, then, is the 

 condition at the centre of curvature, to which such mirror must be brought in order 

 to converge parallel rays Avith exactness. In addition, strict watch must be kept. 

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

 with diaphragms of their aberration, and better yet, by observation of the regu- 

 larity of the curve of that apparent solid, Fig. 1(5, seen by the second test. 



This modification of the first test is literally a method of paraboli/ing by measure, 

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

 focus of a zone is. The little irregularities found round the edges of the holes 

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

 show, too, the increased optical or penetrating power that is gained by increase of 

 aperture. Minute peculiarities, not visible under very high powers with a 10 inch 

 diaphragm, become immediately perceptible even with less magnifying when the 

 whole aperture is used, provided the mirror is spherical. 



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 proper position. The best 

 method for ascertaining its location is, having received the image into the eye, 

 placed purposely too near the mirror, to cause the screen to move across the cone 

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



at the same time, and in the same direction 

 across the mirror. If the eye is then moved 

 from the mirror sufficiently, this black shadow 

 can be made to originate by the same motion 

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

 site side of the mirror. Midway between these 

 extremes there is a point where the advance is 

 from neither side. This is the true position 

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

 perfections of the surface in highly exaggerat- 

 ed relief, as in Fig. 20, which represents the 

 appearance of Fig. 12. 1 



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 he bears in mind the fact that the surface under 



Fig. 20. 



Adjusting the Opaque Screen. 



1 In order to examine Fig. 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. 



