194 PHOTOGRAPHIC PHOTOMETRY. 



day. One might siip[)Ose that such a prodij^ious loweriiii;' of the illumi- 

 nation Avould i)roduce apparently total darkness. Yet our organ takes 

 on such increased sensibility nnder those circumstances that we can 

 not only lind our way about, but distinguish objects, make out details, 

 and even enjoy the landscape. Nay, in some tine tropical regions bright 

 nights seem almost like day. 



Before quitting the subject of the moon let us say a word about tlie 

 earth light. Everybody knows the appearance of "the new moon m 

 the old moon's arms." It is a faint light on the darker side of the moon 

 showing the main features, i. e., the great " seas" or dark patches, to the 

 naked eye, and m a large telescoj^e showing almost everything that can 

 be seen at full moon, though not so well as at other phases. 



The genius of Lionardo (less correctly Leonardo) da Vinci divined 

 that the cause of this was the light reflected from the earth. Photo- 

 grapliic photometry can give us the relative brightness of this light. 

 In an experiment made with a telescoi)e of 0.50 meter a|)erture and 1.00 

 meters focal length I obtained an image of the lunar ball, rendered 

 visible by earth light in sixty seconds, showing the great accidents of 

 its surface. On the other hand, from a series of photographs of the full 

 moon it was found that one obtained by an exposure of one-eightieth 

 of a second had the same intensity. We infer that the ratio of intensity 

 of full earth light to sunlight at the moon's surfjice is 50-00- ^I'fiS*^ I'^^d 

 found jo-oi, in one observation, i^oVo in a subsequent one. The value 

 501)0 seeins probably nearer the truth. These determinations are to be 

 regarded as merely preliminary to a thorough study in which the situa- 

 tions of the three luminaries are to be taken account of and the specular 

 distinguished from irregular reflection from the earth. 



Fixed stars in the focus of a telescope appear nearly like points, not 

 lending themselves advantageously to photometric comparisons, least 

 of all by the photographic method. Accordingly I have ])ropose(l, 

 instead of putting tlie sensitive i)late in the focal plane of the instru- 

 ment to place it a little forward of that plane. In this way, instead of 

 a point each star nnikes a little disk bounded T)y the section of the 

 cone of rays from the objective by tlie ]>linie of the fllni. With a well- 

 corrected lens this disk will be uniformly illuminated. 1 term this disk 

 the "stellar circle." With ftrst-magnitude stars and a telescope of mod- 

 erate size a few secoiuls suffice for obtaining such a photograjjli as is 

 most suitable. We can obtain on one and the same i^late a series of 

 such circles of graduated exposures. 



Forming a similar series with a second comparison star, the differ- 

 ences in the times ot*exi)OSure being constiint in each series, though 

 not quite equal in the t-wo, it will only remain, after the development, to 

 pick out two circles, one from each series of equal intensities, just as 

 in reading a vernier we find two coincident lines. The photographic 

 luminosities of the two stars will be inversely as the times of exposure. 



When the problem is to compare a star with the sun a special device 

 is required on account of the euormous light of tlie letter. We cap 



