364 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1910. 



observe their deeds and manners Lowell has chosen on the Arizona 

 Plateau a privileged site near the route of the Santa Fe, a short 

 distance from the Grand Canyon of the Colorado River, the marvels 

 of which we could not tell here without departing from the purpose 

 of this article. A dry and desert climate, high altitude, and distance 

 from the smoke of manufactories, all contribute to give beautiful 

 images in their telescopes, and they are indeed so very often. 



At Flagstaff they employ chiefly an equatorial of O.Gl-meter aper- 

 ture and 9.45-meters focal length and propose, besides direct visual 

 observations, to obtain photographs as perfect as possible of the gi-eat 

 planets. Lowell noted that generally the definition of the telescopic 

 images is vitiated by the undulations in the lower layers of our atmos- 

 phere whose wave-lengths are of about the order of magnitude of the 

 diameter of the object glass, although often smaller. With such dis- 

 turbances it often happens that the bundle of light waves having 

 traversed regions differently agitated produce a general blurring of 

 the image which is not compensated by a smaller diffraction. Lowell 

 therefore thinks it preferable to diaphragm his objective to 0.30 or 

 0.45 meter. Each of the exposures, which are made in series, receives 

 a bundle of these varying rays which have followed very closely 

 neighboring paths and traversed practically the same atmospheric 

 course. Accordingly the images are very sharp, especially as care is 

 taken to choose the most favorable moments. However, this is not all 

 there is to be said. The form of the diffracted bundle of i-ays is that 

 of a sugar loaf (conical) , so that the diffraction of the image formed 

 of a point is smaller the closer to the summit we cut the cone; or, 

 practically, if we absorb a part of the light by interposing a screen 

 before the plate we will diminish the harmful effect of the dif- 

 fraction, thus improving the definition of the image. This hap- 

 pens very fortunately, for in the use of isochromatic. plates with an 

 ordinary objective corrected for visual observation, we have already 

 stated that it is necessary, in order to obtain a sharp image, to absorb 

 the blue rays by the use of a yellow screen. 



Lowell and his assistants, Slipher and Lampland, have thus been 

 able to photograph for the first time the " canals " of Mars, and, in- 

 deed, so they think, to discover new ones of recent formation. To 

 continue with the suggestive deductions which have led them to con- 

 clude the existence on our neighbor of intelligent beings — indeed, yet 

 more, of consummate agriculturists ! — would take us too far, especially 

 since we would then treat with theories which are strongly contested 

 and upon which we must say that very little light has really been cast. 

 They have made other spectroscopic researches, which have led 

 Lowell to affirm the presence of the bands due to water vapor in the 

 red portion of the spectrum of Mars, a fact favorable to the exis- 



