June 23, 1S99.] 



SCIENCE. 



859 



cent work of Professor George A. Hill with 

 the prime vertical transit. There can be 

 no question of the zeal and industry with 

 which Mr. Hill has for.five years continued 

 a series of observations bearing on one of 

 the most important problems in exact as- 

 tronomy with which we are dealing to-day. 

 Yet the results of his work so far as pub- 

 lished show now and then anomalies and 

 irregularities leading to the suspicion that 

 there is something wrong about the instru- 

 ment. The cause can be found only by crit- 

 ical investigation. It would certainly be very 

 regrettable if such rare qualities as those of 

 Mr. Hill should fail to be productive of their 

 best results through adverse circumstances 

 which would be speedily remedied under a 

 proper system of administration ; and we 

 hope the Committee will either demonstrate 

 that the suspected defects of the work are 

 unreal, or show their cause if thev exist. 



TEE DIFFE ACTION PROCESS OF COLOR 

 PHOTOGRAPHY. 



The prodviction of color by photography 

 has been accomplished in two radically 

 different ways up to the present time. In 

 one, the so-called Lippman process, the 

 waves of light form directly in the pho- 

 tographic film laminte of varying thickness, 

 depending on the wave-length or color of 

 the light. These thin laminae show inter- 

 ference colors in reflected light in the same 

 way that the soap bubble does, and these 

 colors approximate closely the tints of the 

 original. The technical difficulties involved 

 in this process are so great that really very 

 few satisfactory pictures have ever been 

 made by it. The other, or three-color pro- 

 cess, has been developed along several dis- 

 tinct lines ; the most satisfactory results 

 having been produced by Ives with his 



stereoscopic 'Kromskop,' in which the 

 reproduction is so perfect that in the case 

 of still-life svibjects it would be almost im- 

 possible to distinguish between the picture 

 and the original seen through a slightly 

 concave lens. The theory of the three- 

 color method is so well known that it will 

 be unnecessary to devote any space to it, 

 except to remind the reader of the two 

 chief ways in which the synthesis of the 

 finished picture is efl^cted from the three 

 negatives. We have, first, the triple lan- 

 tern and the Kromscope, in which the syn- 

 thesis is optical, there being a direct addi- 

 tion of light to light in the compound col- 

 ors, yellow being produced, for example, 

 by the addition of red and green. The 

 second method is illustrated by the modern 

 trichromic printing iu pigments. Here we 

 do not have an addition of light to light, 

 and, consequentl}', cannot produce yellow 

 from red and green, having to produce the 

 green by a mixture of yellow and blue. 

 Still a third method, that of Joly & Mc- 

 Donough, accomplishes an optical synthesis 

 on the retina of the eye, the picture being a 

 linear mosaic in red, green and blue, the 

 individual lines being too fine to be dis- 

 tinguished as such. 



The diffraction process, which I have 

 briefly described in the April number of 

 the London, Edinburgh and Dublin Philo- 

 sophical Magazine, is really a variation 

 of the thi^ee-color process, though it pos- 

 sesses some advantages which the other 

 methods do not have, such as the complete 

 elimination of colored screens and pigments 

 from the finished picture, and the possibility 

 of printing one picture from another. The 

 idea of using a diffractioa grating occurred 

 to me while endeavoring to think of some 

 way of impressing a surface with a struc- 

 ture capable of sending light of a certain 

 color to the eye, and then superposing ou 

 this a second structure capable of sending 

 light of another color, without in any way 



