1910] on Lowell Olservatory Photographs of the Planets. 821 



to perhaps the greatest mind of the last century, your own Clerk- 

 Maxwell. This, then, was the explanation : in the dusky band we 

 were looking through the crepe-ring on to its own shadow thrown 

 upon the ball. Thus the crepe-ring revealed its presence unmis- 

 takably, not by being seen, but by being seen through. 



If now we compare these images of November 4 with those taken 

 by Mr. E. C. Slipher on September 9 we note a marked contrast in 

 the two fringes of shadow. This corroborates what we have just 

 deduced, for at this time the relative positions of the sun and earth 

 were reversed. 



In the case of Satiu-n, we have as another interesting detail, the 

 excellent instance it aifords of contrast. From the bright equatorial 

 lielt, the most l)riUiant part of the whole picture, we notice a regular 

 gradation of tints down to the faintest parts of the rings. For it is 

 noteworthy that the dark belts of the planet are not so dark as these. 

 This grading is particularly serviceable for being practically that of 

 the eye. For the colour screen and plate used were such as to give 

 us the light from that portion of the spectrum of which the eye takes 

 greatest cognizance. The relative effect, therefore, on the plate is the 

 same as on the retina. 



Lastly, we come to what is one of the greatest triumphs of the 

 whole process : the self-recording of the wisps of Saturn. It was in 

 September that these wisps were first detected visually, independently, 

 by my assistant, Mr. E. C. Slipher, and myself. Curiously enough 

 they were suspected synchronously on the photographic images, and 

 on later ones were definitely seen. They counterpart almost precisely 

 those of Jupiter, though of course in very faint replica. Here comes 

 in the beauty of the photographic method. Instead of taking but a 

 single image, twenty or more are taken one after the other on a single 

 plate. Meanwhile the colour-screen is moved. Thus any detail in 

 the image due to defect on the plate proclaims its origin by its singu- 

 larity, and in the same manner the colour-screen betrays its self- 

 written markings. If a detail is repeated on several images in place 

 it must be real, however faint. 



As we take our leave of Saturn let me point out the beautiful 

 elliptical figures of the rings thus shown, a symmetrical correctness 

 wonderfully pleasing to the eye, and which the best of drawings fails 

 to reproduce. 



From the detail these photographs have thus proved themselves 

 able to depict, they mark a new departure in planetary research. 

 While, on the one hand, they exhibit to the world at large something 

 of the advance recently achieved in our knowledge of the solar system : 

 on the other, they constitute in themselves the beginning of a set of 

 records in which the future of the planets may be confronted with its 

 archived past, and which shall endure after those who first conceived 

 such registry shall have long since passed away. They can never 

 take the place of first-rate visual observation, but they will form a firm 



