550" PROCEEDINGS OF THE SOCIETY. 



diatoms in deposits of similar age in Italy. It was not an uncommon 

 tiling to find tliat other fossils made of carbonate of lime had it 

 replaced by sulphide of iron. Phosphate of lime was often also replaced 

 by suljihide of iron and the interstices of other fossils were often found 

 filled with this same substance which was an exceedingly common 

 mineral in the London clay. Silica was not the difficultly soluble 

 substance which it was formerly thought to be, so that its j)lace could 

 be filled up by any other mineral which was less soluble than itself; 

 and he thought the whole thing might be very fairly explained on 

 the principle of pseudomorphism. But w'hen they came to the 

 question of antiquity, it was not so easy to give an opinion as to 

 whether Count Castracane's diatoms named from the carboniferous 

 series were really of that period. In the Tertiary deposits of course 

 diatoms were found ; but if Count Castracane's determinations held 

 good, they ought to be found in the intermediate series of the 

 Secondary age. 



Mr. Shrubsole said Mr. Kitten's idea was that Count Castracane's 

 fossils were fresh-water diatoms which had got washed down into the 

 coal beds. 



The President stated that there were fresh-water deposits where 

 there were coal beds, and that he did not think much of the objection. 



Mr. Crisp explained, by means of a black-board diagram, the 

 action of the vertical illuminator when used with a transparent 

 object adhering to the cover-glass. 



Mr. Powell said he quite agreed that it was not opaque illu- 

 mination. 



Mr. Stewart exhibited and described the transverse section of an 

 Indian wood having the native name of " Muhkta." It showed alternate 

 layers of xylene and phloem separated by layers of fundamental 

 tissue, the structure being due to the j)eriodic re- formation of cambium 

 from the superficial fundamental tissue. 



Dr. J. Anthony's paper " On Sliding Stage Diaphragms" was read 

 by Mr. Crisp, diagrams in illustration being enlarged upon the black- 

 board (see p. 520). 



Mr. Beck inquired if it was proposed to put the piece of card 

 under the object ? 



Mr. Crisp said that that was so ; in recent years the tendency had 

 been to bring the diaj^hragm nearly flush with the stage. 



Mr. Ingpen was glad that tlie utility of small diaphragms placed 

 closely below the object was being recognized in this country. On 

 the Continent this method (known as the " Oberhanser system") was 

 ])referred to the wheel of apertures at a fixed distance. A dia- 

 l)]iragm, having an aperture of about the diameter of the field of 

 view, was placed close underneath the object, or removed a small 

 distance from it according to the quantity of light required. This 

 was exactly the same as " Varloy's dark chamber," devised by 



