Septembek 5, 1902.] 



SCIENCE. 



369 



it may not be wise to let them go ; the calcu- 

 lations and observations necessary to keep 

 track of them are considerable and ex- 

 pensive. 



This method by the use of larger lenses, 

 longer exposures and more sensitive plates, 

 may show thousands of little bodies, circu- 

 lating not only between the orbits of Mars 

 and Jupiter, but even between the orbits of 

 all the other planets. 



If celestial photography had been known 

 in 1846 and previously, then the discovery 

 of Neptune would have been made by Chal- 

 lis at Cambridge, England, with great ease. 



It was by photography that Herr Witt 

 in 1898 discovered that most interesting 

 minor planet named by him Eros. This is 

 the first body whose orbit has been proved 

 to lie mainly within the orbit of Mars — 

 moving in such a path that at perihelion the 

 earth and planet are separated by about 

 15,000,000 miles. Here, then, we have a 

 grand opportunity for determining its 

 parallax and so getting a new value of the 

 sun's parallax, and hence its distance in 

 miles. Such use has already been made of 

 Eros. A large number of observatories took 

 in 1901 photographs of the planet, and 

 these are to be measured and reduced. But 

 Eros will be better situated in later years, 

 so that during the twentieth century the 

 sun's distance will be obtained with great 

 accuracy. To-day we know that distance 

 with an uncertainty of about 150,000 miles 

 — at the end of the century the uncertainty 

 oiTght to be reduced to 25,000 miles or less. 

 Under the most accurate methods of the 

 present day base lines on this little earth 

 can be measured with an error of even less 

 than one part in a million — or one inch in 

 a million inches, i. e., one inch error in 

 measuring a line nearly sixteen miles long, 

 or half an inch error in measuring a line 

 nearly eight miles long. 



Such accuracy we can hardly hope to 

 reach during the twentieth century in 



obtaining the distance of the sun from the 

 earth. Such an error would amount to over 

 ninety miles. One mile seems a large unit 

 to us, but it is an exceedingly small measur- 

 ing unit for sounding the depths of space. 



The business men of the world are prov- 

 ing to us that there is a great benefit for 

 some one in big combinations of shops and 

 men. The effect in some cases has been to 

 improve machinery, better the output and 

 to reduce prices. This idea of cooperation 

 has taken hold of the scientific mind. To- 

 day seventeen observatories are engaged in 

 making maps of the heavens by photog- 

 raphy. Seventeen observatories from Pin- 

 land to the Cape of Good Hope have been 

 busy for the past ten years in obtaining 

 the images of stars on the sensitive plates. 

 Their plan is a most interesting one for the 

 astronomer. It was arranged by confer- 

 ences of astronomers who met several times 

 at Paris. The heavens have been divided 

 into belts parallel to the equator, and each 

 observatory photographs one or more belts 

 completely around the sky. In order to 

 guard against error a peculiar system has 

 been adopted; each plate is exposed for 

 twenty seconds, the telescope in the mean- 

 time following the star with great nicety. 

 Then the plate is moved a trifle and another 

 exposure is made lasting three minutes, 

 and in a similar way a third exposure is 

 made for six minutes. These three images 

 of a star are very close to each other. 

 Every bright star will make three images. 

 The faint stars will give only two images 

 and the very faint stars one image. This 

 enables the astronomer to judge of the 

 brightness of the stars, and also to discrim- 

 inate between defects on the plate and real 

 images. In order to tie a plate to its neigh- 

 boring plates they are made to overlap, so 

 that twice the number necessary to once 

 cover the sky is taken. This makes 22,000 

 plates. Many of these have now been made 

 and the plates have been measured to de- 



