242 PK0CEED1NGS OF THE SOCIETY. 



night sun. This was offered for distribution amongst the Fellows of the 

 Society, and portions could be obtained on application to Mr. Parsons. 

 The thanks of the Society were voted to Mr. Hilton. 



The President said he had great pleasure in introducing Dr. A. 

 W. Rowe, F.G.S., who had kindly consented to give them a demonstra- 

 tion " On the Photomicrography of opaque objects, as applied to the 

 delineation of the minute structure of chalk fossils." Dr. Rowe had 

 published some very important memoirs on ' The Zones of the Chalk,' 

 and had examined it very carefully all round the coast from Beer Head, 

 Devonshire, to Flamborough Head, in fact wherever it was exposed, and 

 he very strongly recommended his published memoirs to the attention 

 of the Fellows who take an interest in geology. 



Dr. Rowe said he had been invited to come before the Society on 

 that occasion to demonstrate upon the screen some of the possibilities 

 of photomicrography, and as a means of exhibiting the structure of opaque 

 objects of small size but possessing considerable details of a very 

 instructive although very minute character. As regarded photomicro- 

 graphy of transparent objects itself, most people now know something 

 about this, and if they did not practise it themselves they were quite 

 conversant with the results of the process. Great attention had been 

 given to it of late years, so that it might now be said to have become a 

 finished art ; opticians vied with one another in producing lenses and 

 apparatus of the highest excellence, and the process had become not only 

 greatly simplified, but capable of producing extremely beautiful effects. 

 The photomicrography of opaque objects was, however, quite a different 

 matter from that of transparent objects, for though its broad principles 

 seemed to be simplicity itself, as soon as they began to try it they would 

 find that it was beset with difficulties quite unknown to those who had 

 only practised transparent photomicrography. In the production of the 

 photographs exhibited that evening, he had used objectives of various 

 powers from lj in. to 6 in. according to the size of the object or the 

 details he wished to show, and in photographing some of these he used 

 a long camera with a Zeiss' planar lens. As regarded light, there was 

 considerable choice, lime-light, electric, acetylene gas, and others, but 

 he had ultimately fallen back upon incandescent gas-light as being, upon 

 the whole, the best for the purpose. In practice, it was soon found 

 that the ciuestion of success entirely hinged upon getting a good con- 

 trast of light and shade, the whole thing being in fact a matter of tricks 

 of illumination. As regarded time of exposure, with a 3 to 10 mm. 

 diaphragm, he found that a good picture could under favourable con- 

 ditions be obtained in 20 seconds, but that this might extend to as much as 

 3 or 4 minutes at the outside, for specimens which were not very white. 

 He found that the light from the mantle of an ordinary incandescent 

 gas-burner gave a good margin to work with, which was very useful in 

 cases where the plate was under-exposed ; mere speed was no object. In 

 addition to the difficulty in getting exactly the right position for the 

 best effects of light and shade to be produced, a great obstacle arose 

 from the fact that the objects to be dealt with were not flat, causing 

 considerable trouble both in the matter of focussing and of the incidence 



