618 



SCIENCE. 



[N. S. Vol. XII. No. 304. 



musical beats, there is a crowding of the 

 lines ill some parts of the field, alternating 

 with a paucity in intermediate parts, if both 

 gratings be uniform, plane and alike. If 

 the drift of the wires in the two gratings be 

 in slightly different directions, the inter- 

 lacing is dense in the former case and light 

 in the latter, with a diagonal trend. If the 

 gratings be imperfect or not plane, the zones 

 of light and shade must obviously be curved. 

 Even with parallel and equal systems in the 

 same plane, water line effects may be pro- 

 duced, since there is less darkness in the 

 loci where lines cross than where they are 

 distinct. 



WHAT ARE THE GENEEAL PHENOMENA ? 



2. This is all simple enough ; if, however, 

 the two gratings are placed at a distance 

 apart along an axis, and the first illumi- 

 nated by strong diffuse light, the second will 

 project a real image of the former grating 

 at definite points on the axis, almost as if 

 it were a zone plate. When these images 

 are looked at by the eye in the proper posi- 

 tion, they appear as magnifications of the 

 first grating, oftentimes enormously large 

 the size increasing with the distance of the 

 focal plane from the projecting grating. If 

 the eye be moved along the axis the images 

 vanish rapidly to infinity on the nearer side 

 and more gradually to zero on the farther 

 side. Distant foci are apt to show heavy 

 blue lines on a red ground, and vice versa. 

 The indefiniteness of focus when viewed by 

 the normal eye is due to its power of ac- 

 commodation, and the size is an illusion; 

 for the eye is adjusted for an infinite dis- 

 tance and locates the image of unknown 

 position there. The eye unaided is there- 

 fore not well adapted for observations of 

 this character. If, however, one throws the 

 eye out of range with a reading glass of, 

 say, 10 cm. focal distance held close to ifc^ 

 the variability of focal distance is practi- 

 cally wiped out, and the positions of the 



images may now be charted satisfactorily. 



Some years ago, while looking through 

 an ordinary door screen at the Venetian 

 blinds on the opposite side of the street, I 

 noticed that the zones of light and shade 

 were remarkably distinct when viewed by 

 the naked eye (which in my case is near- 

 sighted), but that they all but vanished or 

 were so faint as not to be an annoyance 

 when viewed through spectacles. Tliis ob- 

 servation is general : If the normal eye is 

 put out of proper function by looking 

 through strong convex or strong concave 

 glasses, in either case the shadow zones at 

 tlie proper distance from the screen become 

 painfully pronounced. They disappear as 

 the eye is properly equipped, naturally or 

 otherwise, for long range vision. It seems 

 probable that this principle (to which I 

 shall return in § 5) could be used practically 

 in fitting the eye with the proper glasses. 



For the present purposes therefore either 

 a convex or a concave lens will be needed 

 by the normal eye to fix the propt'r focal 

 planes of the grating ; but as the plane for 

 the convex lens is in front of the eye, this 

 is the more serviceable. Direct projection 

 is only possible in a darkened room and 

 at the strongest focus, supposing that dif- 

 fuse daylight illuminates the first grating. 

 With sunlight all the real foci may be pro- 

 jected, but the use of sunlight (at the out- 

 set) slightly alters the conditions. Foci 

 may also be found hy the telescope directed 

 along the axis ; though furnishing admirable 

 qualitative results, this is the least accurate 

 of the methods and useful only for finding 

 virtual foci in the cases discussed below, § 5. 



Thus the following simple arrangement 

 is suggested for measurement. Along the 

 axis LL' there is placed the ground glass 

 screen C',and the wire gauze* grating^ just 

 in front of it. At a distance, x, from A the 



* Ordinary door screen wire ganze, say 6 inches 

 high and 12 inches wide, in a wooden frame, answers 

 all purposes. 



