Royal Microscoj^ical SocieUj. 5 



pronounced spherules; but, as they brighten in bisulphide when 

 beyond the focus and disappear within, they must be attributed to 

 concavities, openings, or more probably notches in the marginal 

 structure ; but, with respect to the latter, I speak with much 

 reserve. 



If anything more were necessary to prove that this diatom is not 

 made up of bosses it would be found in the fact that the line of 

 fracture in this layer is invariably through the cells. 



The appearance of the cellular or outer layer, here described, is 

 that presented when the inner layer is removed, and when therefore 

 the well-known " eye spots " are wanting. 



When the inner or " eye spot " layer is in situ an " eye " is seen 

 through each hexagonal cell of the outer layer, the hexagonal areolae 

 of the former corresponding in size and position with the hexagonal 

 cells above them. 



In a few cells around the central point of the disk there is a 

 departure from the ordinary form, and the " eye spot " is excentric, 

 sometimes so much so as to appear wanting. 



Although in , these imperfect observations I have carefully 

 avoided generalizing, I may mention that a broken Aulacodiscus, 

 and two or three other discoid forms on the same slide, show an 

 inner plate of similar structure, but it is not thence to be inferred 

 that even in the discoid forms this is universal. 



It seems to me quite possible, if not probable, that some animal 

 tissues, deficient in selective power of staining, may be made to 

 disclose their secrets to the student of minute anatomy, if examined 

 in such media as bisulphide of carbon or oil of cassia. 



I have felt considerable hesitation in bringing this matter before 

 the Society, partly because my knowledge of the Diatomaceae is very 

 hmited, and partly because I was unable to illustrate the appear- 

 ances presented under the microscope ; but my friend Mr. Stewart, 

 to whom my best thanks are due, having kindly volunteered to 

 make the necessary drawings, which he has done with great care 

 and skill under an excellent immersion '-^^ih. by Gundlach, the latter 

 difficulty was overcome. As the chemist is able, by his reagents, to 

 determine the composition of the various substances submitted for 

 his investigation, by the effects they produce, rather than by mere 

 taste, colour, or smell, so, in my opinion, ought the microscopist to 

 be able to determine, not only the form but also the substance of 

 many organisms which he examines, by means of the optical effects 

 produced on transmitted light in different media, rather than by 

 the too often illusory appearances presented without such internal 

 and external aids. 



