Mr. C. J. Burnett on several Forms of Actinometer. 59 



long slip) giving the average or mean for each day of twelve or 

 twenty -four hours*. 



In conclusion, I would call attention to the great importance 

 of carrying on a system of actinic registry coupled with simul- 

 taneous observations of other kinds, including ozone observations 

 (though I believe there is much uncertainty as yet attached to 

 the latter f), (1) in various latitudes, and with reference to the 



* Divisions by tens instead of twelve, might in some respects be pre- 

 ferable, and sets of slips might conveniently, for several reasons, be all of 

 one length ; among other advantages of this uniformity, we could use the 

 same papers for all. An interesting set of observations would be on the 

 actinisms before and after sunset, and on the effects of moonlight (and, 

 query, starlight ?) ; but then we should require either to observe the effect of 

 longish periods, or to employ very sensitive papers, or daguerreotype, or 

 else to concentrate the rays on the solarization-slit by a powerful lens. 

 As to the probable superiority of either mode of estimating actinism, i. e. 

 by the amount of its action on sensitive paper, or by that on sensitive 

 liquids (or, it may be, on gases, such as carbonic oxide, and chlorine, &c), 

 their relative advantages will probably be found to vary considerably ac- 

 cording to varying circumstances. For a continuous observation there are 

 certainly (unless we use more complicated apparatus) some apparent ad- 

 vantages attending the estimation by sensitive paper alone ; and in many 

 cases it might be found possible to preserve a tolerably accurate record of 

 the relative depths of tint produced (without the trouble and risk attendant 

 on keeping, hypo-fixing, and printing from the impressed papers), by pro- 

 viding ourselves with a scale of permanently tinted paper, ivory, or other 

 material representing a great variety of depth of tint gradually increasing 

 from one end of the scale to the other, and each depth of tint being marked 

 with a number, 1, 2, 3, 4, &c. We should only (after some slight and tem- 

 porary fixing process, sufficient to prevent rapid action of light during com- 

 parison) require to shift each part of the actinized paper along the scale till 

 we came to a part corresponding to it in depth of tint, and then record the 

 number (1, 2, 3, or others) which represents it. There is, however, a diffi- 

 culty attending this plan — that the colour of the actinized paper may turn 

 out to be different from that of the tints on our scale, so that it may not 

 be found easy to say what is the depth of tint corresponding. This diffi- 

 culty, however, might be met or obviated in various ways. 1. By having 

 several similar scales of different colours, such colours being the same as 

 those which our sensitive paper is found liable to assume. 2. By employing 

 such photographic papers, &c. (developments if necessary), whether argen- 

 tine, ferric, uranic, chromic, or others, as are least liable to vary in colour, 

 either when used on different occasions or on one occasion in the one slip, 

 according to strength of actinism. 3. By making even our one scale vary 

 to a certain extent in the nature of the colour, as well as in depth of it, if 

 the difficulty just alluded to in connexion with this should still be found 

 troublesome. My impression, however, is that there are photographic 

 papers which will be found to colour so uniformly as to render this varia- 

 tion on our scale unnecessary. 



t How, for instance, can we distinguish between the effects of loosely 

 absorbed actinism and ozone ? and how are we sure that the actions of the 

 tests themselves may not produce it, and more rapidly in certain states of 

 the air than in others ? 



