212 Principles of Photography. 



OCCULTATIONS. 



This month affords several of these phenomena, which 

 should not be neglected by the observer. The first, on 

 Oct. 8th, will be an interesting one as regards the object, and 

 under favourable lunar circumstances, near the time of the 

 quadrature, when a good telescope may still be expected to 

 show the dark limb, p 2 Sagittarii, 5| mag., will disappear at 

 7h. 17m., and be followed by its more brilliant companion, at 

 a little distance N, p 1 , 4 mag., 4m. afterwards : the respective 

 emersions will be at 7h. 48m. and 8h. 8m. The next, on the 

 9th, is of a yet larger star, /3 Capricorni, 3 mag., which will 

 be hidden from 8h. 35m. till 9h. 28m. 14th, e Piscium, 4 mag., 

 from 6h. 59m. to 7h. 48m. 



Addenda. — In the last number of the Intellectual Ob- 

 seeveb it was stated that ' ' sanguine stars are, without excep- 

 tion, so far as is hitherto known, insulated." I have since, 

 however, found an instance in Sir J. HerschePs Cape Observa- 

 tions, where two stars, each 9 mag. and 10" apart, are both 

 " of a full scarlet colour." They form his No. 4649. 



Those who have felt interested in the speculations as to 

 the constitution of the sun and stars, referred to in the same 

 paper, will rejoice to hear that Professor Donati has succeeded 

 in seeing and measuring spectral bands in the light of the 

 late comet. If these can be identified with those of any 

 known elements, it is the greatest step that has yet been 

 made towards a knowledge of the real nature of these most 

 mysterious bodies. 



PRINCIPLES OF PHOTOGRAPHY.* 



BY J. W. M f GAULEY. 



Management of the Light. Photography is based on the 

 chemical or actinic action of light ; the luminous rays would be 

 inert, or even mischievous, but for the actinic rays present along 

 with them. The green tint found in objectives, especially those 

 of crown glass, lengthens the time required for exposure ; but, 

 as it affects the strong central rays most, it tends to equalize 

 the action of the light over the whole field. There is reason 

 to believe that the rays which traverse the thick part of tho 

 lens are thus deprived of half their actinic power. Much, 

 however, depends on the kind of green that constitutes tho 



* Tins is the third article of the series, of which the fourth and concluding 

 paper will shortly appear — the first, on " The History of Photography," is in 

 Ho. 27; and the 6ccond, on "Photographic Processes," in No. 28. 



