1881.] Spectrum on the Haloid Salts of Silver, fyc. 16& 



gators are compared one with another there are often very wide 

 discrepancies apparent. It appeared to me that it was desirable if 

 possible to examine the subject afresh, and to endeavour to reconcile 

 or to explain as far as possible these discrepancies. 



The earlier investigators, such as Herschel, Hunt, Draper, and 

 Becquerel, added much to the knowledge of the subject, but their 

 researches were carried on at a time when the modern modifications 

 and more powerful means of development of an image were unknown. 

 Later investigators, including such eminent names as H. W. Yogel 

 and Eder, have availed themselves of the modern appliances, but their 

 results are not always consistent one with another. 



In the following researches points are brought out which are, it is 

 believed, new and deserving of attention, not only on account of their 

 applicability to the practical working of photography, but also because 

 they throw a light on molecular physics. 



For solar photography it is essential that a knowledge of the relative 

 effect of the various parts of the spectrum should be known, since, 

 if the photo-heliograph be adjusted for one particular part, and 

 the films employed be more sensitive to another part, it is manifest 

 that no great sharpness of image can be obtained. The following 

 researches it is believed show what an enormous effect the mixture of 

 haloid salts has in shifting the position of maximum effect, and it may 

 be possible to either alter the achromatism of the objectives employed,, 

 or else solely to use the sensitive compound to which the objective is 

 at present adapted. 



Apparatus employed. — The spectroscope employed in these researches 

 was that already described " On the Effect of the Atomic Grouping of 

 Molecules." * Two prisms of medium dense and colourless flint- 

 glass were used to obtain the necessary dispersion. They were set 

 to have the angle of minimum deviation near Gr. The angle of disper- 

 sion between A and H was about 6^°, the length of spectrum between 

 these two lines was about 2^ inches, the spectrum in the ultra-violet 

 extending some 1^ inch beyond H, and the infra-red about f of an 

 inch beyond A. The whole spectrum as given by the prisms under 

 consideration thus had a length of 4^ inches, a length in which all 

 phenomena could be fully recognised and measured. 



Sources of Light. — The sources of light employed were the sun 

 and the crater of the positive pole of the electric light. Images of 

 these sources were thrown on the slit by means of a condensing lens 

 alone in the second case, and by it and a heliostat in the first case. 



Vehicles holding the Sensitive Salts. — The sensitive salts were held 

 in situ, in paper, in gelatine, and in collodion, in the last vehicle the 

 salts being prepared either as emulsions in fluid collodion or by the 

 ordinary silver nitrate bath process. In gelatine the salts were all 

 * " Phil. Trans." for 1881 : Part 3. 



