TRANSACTIONS OF THE SECTIONS. 5 



a steel plate, it liquefies, forming an iodide of iron, and a dew spreads 

 around the central point. Now, if this dew is examined in a good 

 microscope, its globules are seen not to be arranged casually, but in 

 straight lines along the edges of the minute striae or scratches which 

 the microscope detects even on polished surfaces. This is another 

 proof how vapour is attracted by sharp edges, for the sides of those 

 striae are such. Whether or not these facts had any relation to that 

 observed by M. Daguerre, of the action of vapour at an angle of 45°, 

 Mr. Talbot did not pretend to say, but thought them worthy of being 

 mentioned to the Section. 



He observed, that it had been repeatedly stated in the Comptes 

 rendus of the French Institute, that M. Daguerre's substance was 

 greatly superior in sensitiveness to the English photogenic paper. It 

 now, however, appeared that this was to be understood in a peculiar 

 sense, inasmuch as the first or direct effect of the French method was 

 very little apparent, and was increased by a subsequent process. This 

 circumstance rendered it difficult to institute a direct experimental com- 

 parison between them. If it could be accomplished, he doubted whether 

 M. Daguerre's substance would be found much more sensitive than his. 

 The present degree of sensitiveness of the photogenic paper was stated 

 to be as follows : it will take an impression from a common argand 

 lamp in one minute, which is visible though weak. In ten minutes the 

 impression is a pretty strong one. In full daylight the effect is nearly 

 instantaneous. M. Arago had stated that M. Daguerre had obtained 

 some indications of colour. Mr. Talbot thereupon referred to his paper 

 to the Royal Society, read January 1839, and published in the Athe- 

 TKBum (No. 588), wherein he had stated the same thing. Since then, 

 more considerable effects of colour have been noticed. In copying a 

 coloured print the colours are visible on the photograph, especially the 

 red, which is very distinct. Some descriptions of photogenic paper 

 show this more than others ; but no means have yet been found of 

 fixing those colours, and sunshine reduces them all to an uniformity of 

 mere light and shade. Sir John Herschel has formed images of the 

 solar spectrum, in which the change of colour is seen from end to end 

 of the spectrum, but most clearly at the red end. Mr. Talbot then 

 mentioned a kind of photogenic pictures which a,fford a very capricious 

 phaenomenon. The objects are represented of a reddish colour on a 

 white ground, and the process leaving the pictures in such a state that 

 they are neither ^a;ec?, nor yet the contrary, but in an intermediate 

 state; that is to say, that when they are exposed to sunshine they 

 neither remain unchanged (as fixed pictures would do), nor are they 

 destroyed (as unfixed pictures would be) ; but this singularity occurs, 

 that the white ground remains unaltered, while the colour of the object 

 delineated on it changes from reddish to black with great rapidity, after 

 which no further change occurs. These facts (he thought) serve to 

 illustrate the fertility of the subject, and show the great extent of yet 

 unoccupied ground in this new branch of science. 



