4G KEPORT— 1872. 



Pliotograpbic Society in Juue of last year. But one difficulty opposed itself to the 

 obtaining of good results -with certainty — the difficulty of coutrolliug the excess of 

 nitrate of silver from lapsing into an over-sensitive state, and thus causing what in 

 photographic ;j«;V««re is called "fog." 



To remedy this state of things, and to have the power of producing a sensitive 

 emulsion that shall keep for months in perfect working order, by adding something 

 to the emulsion that shall exercise a controlling power over the free nitrate of silver, 

 was the problem which the author set himself to work out, and he has been fortu- 

 nate enough to achieve a complete success. 



The author had been familiar with the fact that a mixture of the nitrates of silver 

 and uranium in solution would retain for years their sensitiveness to light without 

 theirgood qualities being in any way impaired; and it occurred to him that the addition 

 of the nitrate of another metal to that of silver in the sensitive bromized emulsion 

 would give us the power which we wanted of being able to keep a large excess of 

 nitrate of silver from the decomposition which apparently resulted in fog. 



In order to make an emulsion collodion which shall have an exalted sensitiveness, 

 and which shall retain all its excellencies unimpaired for months, the author has, 

 after forming therein a certain amount of bromide of silver, add the nitrates of silver 

 and iiraniu.ni 'together to the emulsion in certain definite proportions. The result 

 is the formation of a highly sensitive mixture in which no change whatever occurs 

 for a period of certainly three months ; and this result cannot be obtained by any 

 means other, so far as he is aware, than by the addition of the nitrate of another 

 metal. The author has tried various other nitrates with perfect success, but has 

 selected and recommended the nitrate of m-anium as having, on the whole, gi-eater 

 advantages than anv other nitrate with which he is acquainted. 



This sensitive emulsion is also of very great value for the preparation of sensitive 

 photographic films to be used in a dry state. These films, prepared with a collodion 

 containing bromide and excess of nitrate of silver (the latter being controlled by 

 the presence of nitrate of uranium), can now be prepared with certainty to have a 

 sensitiveness equal to the best wet collodion sensitized in a bath ; and the use of 

 nitrate of uranium gives them the extraordinary advantage of retaining their ex- 

 quisite sensitiveness unimpaired for any reasonable time ; and they will bear after 

 light has impressed a picture upon them the delay of months previous to the deve- 

 lopment of the invisible impression. 



It is with the special object of pointing out how important to the cause 

 of science in distant lands such photogTaphic dry plates ma_y become that the 

 author introduced the subject, as he caimot but feel that if naturalists, geo- 

 logists, and botanists in distant lands can secure records from day to day on sensi- 

 tive photographic plates which need not be developed till they return from their 

 expedition, a new power will be placed in the hands of scientific travellers of which, 

 he thinks, they will not be slow to avail themselves. 



The author is enabled to speak with great confidence on this point, having himself 

 exposed some of these sensitive dry films in the beginning of May of this year, and 

 which have only now (the middle of August) had the latent image developed, 

 and that without any deterioration whatever. 



As, moreover, dry films prepared according to the manner the anther has indicated 

 appear to be entirely tmaft'ected by gi'eat heat, they will be of value in explorations 

 in tropical countries, where anj' other known method of photography would be a 

 great difficulty, if not a real impossibility. 



On tJie VelocUij of Light in tJte Chemical Elements, and on their Crystalline 

 Form. By Ch. V. Zengee, Professor in the Polytechnic School in Prague. 



The theory of vibratory motion is in strict accord with experiment in the case 

 of sotmd and its propagation. It was from the analogy between light and sound that 

 hysicists ascribed the same laws of motion to both, representing their velocities 

 ly the same equation, _ 



d 



