Proceedings of the British Association. 393 



" On a remarkable Photographic Process, by which dormant pictures 

 are produced capable of development by the breath or by keeping in a 

 moist atmosphere,'''' by Sir J. F. W. Herschel. If nitrate of silver of 

 specific gravity 1*200, be added to ferro-tartaric acid of specific gravi- 

 ty T023, a precipitate falls, which is in a great measure redissolved by 

 a gentle heat, leaving a black sediment, which being cleared by subsi- 

 dence, a liquid of a pale yellow color is obtained, in which a further 

 addition of the nitrate causes no turbidness. When the total quantity 

 of the nitrate solution added, amounts to about half the bulk of the 

 ferro-tartaric acid, it is enough. The liquid so prepared does not alter 

 by keeping in the dark. Spread on paper and exposed wet to the sun- 

 shine (partly shaded) for a few seconds, no impression seems to have 

 been made, but by degrees, although withdrawn from the action of the 

 light, it develops itself spontaneously, and at length becomes very in- 

 tense. But if the paper be thoroughly dried in the dark, (in which 

 state it is of a very pale greenish yellow color,) it possesses the singu- 

 lar property of receiving a dormant, or invisible picture ; to produce 

 which (if it be, for instance, an engraving that is to be copied) from 

 thirty seconds to a minute's exposure in the sunshine is requisite. It 

 should not be continued too long, as not only is the ultimate effect less 

 striking, but a picture begins to be visibly produced, which darkens 

 spontaneously after it is withdrawn. But if the exposure be discontin- 

 ued before the effect comes on, an invisible impression is the result, to 

 develope which all that is necessary is, to breathe upon it, when it im- 

 mediately appears, and very speedily acquires an extraordinary intensi- 

 ty and sharpness, as if by magic. Instead of the breath, it may be sub- 

 ject to the regulated action of aqueous vapor, by laying it in a blotting- 

 paper book, of which some of the outer leaves on both sides have been 

 dampened, or by holding it over warm water. Many preparations, both 

 of silver and gold, possess a similar property in an inferior degree, but 

 none to the extent of that above described. 



Dr. Robinson read a paper, " on the Barometric Compensation of the 

 Pendulum.''' 1 At the Manchester meeting of the Association, (1842,) 

 Prof. Bessel made a communication on the improvement of the Astro- 

 nomical Clock, which, with other valuable matter, contained a proposal 

 to compensate for the changes of rate produced by the varying density 

 of the atmosphere. Dr. R.. would not have adverted to the subject, did 

 he not think that a method proposed and applied by him twelve years 

 ago, possessed certain advantages over that of the illustrious astrono- 

 mer of Konigsberg, which entitle it to the preference in practice. As 

 early as 1825, he had ascertained the fact, that the received buoyancy 

 correction was too small, by comparing the rates of a transit clock with 

 the barometric indications ; and Col. Sabine gave the final proof of it 

 Vol. xlvi, No. 2.— Jan.-March, 1844. 50 



