500 THE POPULAR SCIENCE MONTHLY 



Inasmuch as planets, shining by reflected light, do not act upon 

 photographic plates so strongly as stars of the same visual magnitude, 

 we may say that exposures which recorded stars down to the ninth 

 magnitude should have recorded planets down to the eighth. From 

 the known brightness, distance from the sun and approximate diameter 

 of a few of the asteroids revolving in space between Mars's and Jupiter's 

 orbits. Dr. Perrine has computed that an average eighth-magnitude 

 intramercurial planet could scarcely be- larger than thirty miles in 

 diameter and that roughly a million such bodies, of great density, 

 would be required to supply the disturbing effect observed in Mercury's 

 orbit. 



Taking all these points into consideration, 1 think we may say that 

 the investigations by Perrine, forming a part of the work of the 

 Crocker Eclipse Expeditions from Lick Observatory, have brought the 

 observational side of the Intramercurial Problem, famous for a half 

 century, definitely to a close. It is not contended that no such planets 

 will be discovered in the future; in fact, it would not be surprising, 

 nor in opposition to the opinions here expressed, if several such bodies 

 should be found; but it is confidently believed that any such bodies 

 would fail hopelessly to supply the great mass of material demanded 

 by Le Terrier's theory, as Perrine pointed out in discussing the Sumatra 

 observations of 1901. 



On the occasion of a future eclipse of fairly long duration, occurring 

 in the dry season, it might be well to repeat the observations, inasmuch 

 as the instruments are in approximate readiness, and the observations 

 at the three past eclipses were made through thin clouds twice, and 

 with cloud-shortened exposures the third time. The cameras are 

 capable of recording tenth-magnitude stars with three-minute ex- 

 posures in clear sky. It will not be advisable to use these instruments 

 at the eclipses of the next four years. 



There are other chapters, on the theoretical side of the problem, to 

 be entered here. 



Professor Newcomb's researches on planetary motions extended 

 much further than Le Verrier's. He found small terms in the motions 

 of all the inner planetg — Mercury, Venus, Earth, Mars — which are not 

 due to the disturbing attractions of any known masses of matter. The 

 chief discrepancies, aside from the large one found in Mercury's motion 

 by Le Verrier and confirmed by Newcomb,^ are in the perihelion of 

 Mars, and in the nodes of Mercury and Venus. These outstanding 

 residuals will be tabulated on a later page. 



The attractions of any one j^lanet or ring of small planets, suffi- 

 cient to account for the excess motion of Mercury's perihelion, failed 

 to account for the other discrepancies discovered by Newcomb for the 



*Le Verrier's discrepancy amounted to 38", Newcomb's to 41". 



