272 METEOROLOGY AND ALLIED SUBJECTS. 



Fourth. Vogel's chemical photometer. This was especially designed 

 for the use of the photographer, and requires considerable further study 

 before it can be used for accurate scientific i)urposes. The measure- 

 ments are based essentially upon the action of light upon sensitive 

 chrome i^aper. 



Fifth. Draper's tithonometer. This was the first attempt to measure 

 the chemical intensity of light, and its principle is similar to that of 

 Bunsen and Eoscoe's "chlorhydrogen, but it could not yield accurate 

 results. 



Sixth. Becquerel's electro-chemical actinometer. This consists of a 

 jar of water, in which two silver plates are immersed, which are cov- 

 ered with equally thick layers of violet silver chloride. A conductiug 

 wire connects them, and a delicate electric galvanometer is introduced 

 into the current. If these plates are exposed to the light the needle of 

 the galvanometer shows the existence of an electric current. After a 

 careful discussion of the relative merits of these methods Pernter shows 

 that the total chemical intensity can, in no case, be measured absolutely, 

 so long as the apparatus takes account of only one portion of the spec- 

 trum, since it is now known that all the rays of the spectrum have chem- 

 ical effects, j)rovided they fall upon the proper substances. Keeping this 

 in view, as well as on account of its convenience, Pernter maintains that 

 the photographic actinometer is the least objectionable instrument, and 

 has, in fact, a great advantage over the others, in that no absorbing 

 glass or other substance intervenes between the source of light and the 

 sensitive paper. He earnestly recommends, therefore, that for future 

 observations in meteorology, when the object is to determine the rela- 

 tive intensity of the sunlight at given moments, the photographic 

 actinometer be employed, as the chemical preparations are easy to make, 

 the maniijulations are soon acquired, the observations require but a very 

 short time, and the apparatus is so portable as to be available on 

 scientific exijeditions. On the other hand, if the object is to determine 

 the sum total of the action of sunlight during the day or other interval, 

 then the photantitypimeter of Marchand has the advantage, in case the 

 preparation of chloride of iron is intrusted to skillful hands. {Z. 0. G. 

 M., XIV, p. 254.) 



Having given this general review of the methods adopted by investi- 

 gators, Pernter details theresults attained hitherto in the photo-chemical 

 measurement of sunlight. Bunsen and Koscoe, with their chlorhydro- 

 gen photometer, found that the chemical intensity of diffuse daylight 

 (H) is represented by the following formula : 



H = 2.776 + 80.849 cos <p - 45.996 cos V 



But for the chemical intensity (J) of the direct sunlight falling per- 

 pendicularly upon the sensitive plate, they found an exponential form- 

 ula which, multiplied by cos ^, gives the intensity (S) upon a horizontal 

 area at the surface of the earth, and which is approximately as follows : 



