Intelligence and Miscellaneous Articles. 91 



pidity and power. The result of my experience hitherto with this 

 calotype paper is, that if properly prepared, it will keep three or four 

 months, ready for use at any moment, and moreover it is used in a 

 dry state, which is a great convenience. 



The time of exposure to light in the camera may be varied, ac- 

 cording to circumstances, from a quarter of a minute upwards ; and 

 the paper, when taken out of the instrument, appears quite blank, 

 as I said before, but it is impressed with an invisible image. It 

 may be kept in this invisible state for a month or so, if desired, and 

 brought out, or rendered visible, when wished for. But, generally, 

 this is done shortly after, or at least on the same day, for fear of ac- 

 cidents (such as a casual gleam of daylight, which would at once 

 annihilate the whole performance). Whenever it is desired to ren- 

 der the picture visible, this is done in a very short time, as from a 

 minute to five or ten minutes, the strongest impressions coming out 

 the easiest and quickest. Very faint impressions (as those obtained 

 when the paper has been only a few seconds in the camera, or the ob- 

 jects have not been luminous enough) take a longer time in coming out; 

 but they should not be despaired of too soon, as many of them ex- 

 hibit difficulty at first, as if reluctant to appear, but nevertheless end 

 by coming out very well. The operator of course remains in a dark- 

 ened room, lit by candles only. 



I know few things in the range of science more surprising than 

 the gradual appearance of the picture on the blank sheet, especially 

 the first time the experiment is witnessed. The operator ought to 

 watch the progress of the picture, until, in its strength of colour, 

 sharpness of outline, and general distinctness, it has reached in his 

 judgement the most perfect state. At that moment he stops further 

 progress by washing it over with a fixing liquid. This is washed off 

 with water, the picture is then dried, and the process is terminated. 



The picture is found to be very strongly fixed, and from it nu- 

 merous copies may be taken on common photogenic drawing paper, 

 by the method of superposition in sunshine. The original picture 

 does not readily become altered, or wear out by this exposure to the 

 sun ; but in case it does so, as happens sometimes, I find that it may 

 be in general readily revived. This revival, which is a most curious 

 particularity of the calotype process, not only restores the picture to its 

 pristine strength, but frequently causes fresh details and minutiae to 

 appear in the picture, which had not appeared before, at the time 

 when it was first brought out, or rendered visible (owing to that pro- 

 cess having been checked too soon). These details, therefore, had 

 been lying in an invisible state on the paper all this time, not de- 

 stroyed (which is the most extraordinary thing) by so much expo- 

 sure to sunshine. They were protected by the fixing liquid. But 

 no one could have supposed beforehand, or without ocular demon- 

 stration, that it could have exerted so complete a protecting power. 

 This is an invaluable property of the calotype — the power of reviving 

 the pictures — not only because it allows so many copies to be made, 

 but because it enables the artist to correct the error of his judgement, 

 in case he has made too faint a picture at first, by stopping it too 

 soon while it was coming out. 



