816 Professor Percival Lowell [April 8, 



disk of a planet could be put inside the image of a single star. For 

 a like cause reflectors cannot be employed ; for with them all faults, 

 instrumental or atmospheric, are magnified three-fold over those of 

 a lens. They may give imposing-looking pictures, but the finer 

 detail is lost ; a fact which is evident at once to an expert. Now 

 it is in the registration of this finer detail that the accomplishment 

 lies, and which from a scientific point of view marks its importance. 



Study of the conditions leading to definition has made these 

 photographs possible, just as lack of such study alone makes possible 

 the scepticism one sometimes hears. Thus it is a well-known fact 

 with us that the main markings of a disk may come out sharp, while 

 the delicate ones are obliterated by a blur which otherwise eludes 

 detection. This applies as much to photographic as to visual results, 

 and it is this defect that a reflector introduces. Another optical 

 mistake, which has latterly been hailed as showing that the lines are 

 not lines but a series of dots, was made the other day in France. The 

 observer saw perfectly correctly, but one with knowledge of the optics 

 of a telescope in our air should have known that the effect observed 

 was the inevitable result of using an aperture which the seeing did 

 not warrant, as he could easily have assured himself l)y looking at the 

 shattered rings in the synchronous image of a star. Even in our far 

 better Flagstaff atmosphere the best results are got by diaphragming 

 the aperture down. 



In photography we cannot diaphragm down to advantage because 

 we need the light, and this is one reason why photographs cannot 

 rival an expert eye. Visual observations conducted by an eye fitted 

 by nature, and trained by experience, must always surpass the best the 

 camera can do. 



What the new process does do is to monochromatise the light as 

 nearly as possible. This is accomplished by a colour-screen, and a 

 plate sensitised in accord with it. Then at the moment of exposure 

 every precaution is taken that all movement shall be as nearly nil 

 as can be secured within the instrument itself, and in the air Ayithout 

 it. Lastly, he who would photograph the canals most successfully 

 must first have seen them, that he may know when his opportunity 

 arrives. 



Planetary photography is not intended, nor is it destined, to super- 

 sede visual observation. Research on the planets must rest in 

 future, as in the past, on the ultimate power of the eye and of the 

 brain behind it, whether this take the form of telescopic, spectroscopic, 

 or other perhaps new line of inquiry. But in certain ways the sensi- 

 tive plate may supplement the retina. Position is one of these, 

 contrast another. For the eye to place in their proper posts all the 

 markings of a multi-featured disk in the short time at its disposal is 

 a well-nigh impossible task. The film registers them at once in situ. 

 Values are another thing the photographs bring out clearly. They 

 exaggerate contrast, it is true, as compared with the eye : but this is 



