56 Mr. C. J. Burnett on several Forms of Actinometer. 



We now come to the paper actinometer ; which offers the very 

 great advantage that the sensitive substance and the registering 

 material are one and the same. Nothing, in fact, can well be 

 simpler than the apparatus. The sensitive paper, ruled with lines 

 to correspond with the seconds and minutes and hours, hours, 

 days, and weeks, weeks and months, or whatever other intervals 

 are to be observed, is made by clockwork to pass (either firmly 

 fixed and revolving on one roller of large diameter, or otherwise) 

 behind a hole or slit which admits the light, the intensity of 

 which will be of course registered by the differing degree to 

 which the different parts of the paper are affected by it*. 



The main difficulties attending this system of actinometry are, 

 1st, the difficulty of securing uniform sensitiveness in the same, and 

 still more in different samples of paper, even when similarly sen- 

 sitized ; 2nd, corresponding uncertainties in development, where 

 development-processes are used ; 3rd, the difficulty of keeping 

 paper unaltered in its sensitive condition, and this both before 

 and after the solarization (as we may wish to develope several 

 sheets at once, to be sure that the developer acts alike on them) ; 

 and 4th, the difficulty of different papers being differently acted 

 on by the fixing solutions. Still all these difficulties may to a 

 great extent be got over by care and proper contrivances in the 

 selecting, sensitizing, drying, and development of papers, cutting 

 off parts near the edge, and using for each observation, not one 

 single slip, but several, say two or three, cut either from differ- 

 ent sheets or different parts of the same sheet, or, if from origi- 

 nally contiguous parts, with the sides (not the surfaces of the 

 paper) of one of them reversed, so that the original right-hand 

 of the one slip, or its top part, corresponds to, and is, when in the 

 actinometer ,in contact with, or juxtaposition to, what was originally 

 the left-hand side of the other slip, or the lower part of it, if they 

 were cut from top to bottom of the sensitive sheet. 



As to the third source of difficulty attending this variety of 

 actinometer (for in the others, where the paper is only the regis- 

 tering machine, the exact depth of tint produced is of no moment, 

 the direction or curve of its line being all that is required), there 

 would appear to be modes of sensitizing the silver papers which 

 give a much better paper as regards keeping than others. It 



showed little action. The addition of a neutral chromate, however, made 

 them much more sensitive ; and after this addition they were found capable 

 of yielding very sensitive photographic papers, giving good impressions, 

 capable also of further development by silver-salts, metal -cyanogen salts, &c. 

 * To eliminate possible action of actinism held in solution by the air or 

 other actions of the air, it might be well to compare the record of the in- 

 strument when the paper in the solarization-slit is open to the air, with the 

 action when it is protected by a thin scale of glass. This comparison might 

 possibly lead to very interesting results. 



