Mr. C.J. Burnett on several Forms of Actinometer. 57 



may be generally observed that, using the ordinary style of papers, 

 the best plan, where the paper is intended for a long observation, 

 is not to use development, and not to make it too highly sensi- 

 tive, also to sensitize it as if it were a very weak paper for the 

 ordinary positive printing. Where, however, it is intended for 

 registry of seconds or minutes, we must use a more highly sen- 

 sitive printing-paper, or have recourse to an after-development of 

 it ; and it is here also, when we wish to have sets of successive 

 observations made from slips cut from one sensitive sheet (to en- 

 sure uniform value of colour), that our difficulty is felt. 



Suppose we have two, three, or four slips, each one repre- 

 senting, say, a different hour with the variations during it*. 

 Here there may be no very great difficulty in one way, as sup- 

 posing the hours successive, there is hardly time for the paper to 

 go far wrong ; but suppose they are successive days instead, we 

 see the relative value of the periods on each day, but are at a 

 loss to estimate their value compared with another day. The 

 simplest plan which suggests itself is to check the value of the 

 hour observations by day observations, of the day observations by 

 week observations, and so on, — the observations of the longer 

 periods being made, as they conveniently can be, on much weaker 

 and consequently much better-keeping paper; and these dif- 

 ferent sets of observations will enable us to eliminate not 

 only errors produced by the sensitive paper going wrong by 

 keeping, but original differences in the quality of the papers 

 before sensitizing, as well as in the chemicals, and, what with all 

 care is most difficult to avoid, in the manipulation and in the 

 weather. 



As to the fourth difficulty (that connected with the inequality 

 in fixing), with all care and study of time, and keeping the liquid 

 in motion, it is a serious one ; for, though we may secure pretty 

 uniform action by these precautions and by study of strength 

 and temperature in the case of one piece of paper, yet in the 

 case of comparisons between different papers we must trust a 

 good deal to the system of mutual checks (as of day papers or 

 hour papers, &c), as is to be explained. 



Another plan, however, is, to be satisfied with a less perfect 

 fixing, which will do well enough to engrave from, or copy in 

 China ink, or to print from by a negative process, or to make a 

 negative from in the camera. 



For some of our registers by the Papyro- actinometer, the 

 gradually and uniformly progressive motion behind the solariza- 

 tion-slit may be the best ; but for others a motion by jerks, 



* Or, as formerly described, we may and should have two or more slips 

 (say reversed, as described, for each hour) ; but we have left this out for 

 simplicity in the explanation. 



