CORRESPONDENCE. 51 



immersion lenses constructed on certain formulae transmit rays of greater 

 angle than corresponds to the maximum air-angle. 



" Crito " now offers a totally different explanation, ignoring my 

 criticism, apparently unconscious that lie must retract his former 

 fallacious explanation before he is entitled to start afresh on the same 

 subject. 



My observation, that a certain ^V*h lens, when tried by the test 

 of deep oculars, did not give definition beyond about one thousand 

 diameters, seems to have puzzled " Crito," — he cannot realize it. 

 He says the answer to such a statement is " so obvious that the dis- 

 cussion is puerile : the initial power is greater." 



The discussion would indeed be puerile if so obvious an answer 

 had been effective and had escaped my notice. But " Crito " is again 

 in error : it is not true that the initial power of a o^gth is necessarily 

 greater than 1000 diameters. The initial poioer of a lens depends on 

 the length of optical body and on the ocular used. 



In the list of magnifications belonging to the particular stand and 

 oculars (Hartnack's) I used to determine the lunit of definition in 

 Zeiss's lenses, the initial poiver of a -^^th is given as 820 diameters ! 

 In Zeiss's list it is given as 680. 



Your obedient servant, 



John Mayall, jun. 



An Animal-like Diatom. 



To the Editor of the ' Monthly Microsco-pical Journal.' 



Sib, — The diatom figured in ' Nature,' and reproduced in the 

 ' M. M, J., is a Cocconeis, probably the common C. scutellum. Pseudo- 

 podal-like appendages are not a new discovery or peculiar to this 

 form. Ehrenberg gave a figure of a Siu'irella, S. gemma (I think in 

 his ' Infusionsthierchen '), afterwards reproduced by Pritchard in his 

 third and fourth editions of the ' Infusoria,' pi. xii,, fig. 4, surrounded 

 by a fringe of stout setae. Professor Smith remarks (see Pritchard, 

 third edition) that he has often seen >S^. gemma in this condition, but 

 he never detected any motion in these appendages, and considers 

 them to be a parasitic growth. 



I once found Campylodiscus clypeiis ■with' sunilar gelatinous pro- 

 longations attached to the margins o;f the frustules ; when first ob- 

 served they were in a very vigorous condition, but after the lapse of a 

 few weeks the frustules gradually died and the fringe disappeared. I 

 do not think the growth was parasitic, but was an abnormally large 

 development of the mucous envelope which all diatoms secrete in a 

 greater or less degree. I once saw Navicula serians in series like a 

 Fragilaria, the frustules being held together by a mucous envelope. 

 This secretion is sometimes amorphous, with the diatoms scattered in 

 it as in Mastogloia, or it assumes a definite form, as in Schizonema 

 and Encyonema, in which it forms long hair-like filaments, or it is 

 secreted in greater quantity at one end than the other, forming stipes 

 as in Cocconema (I have seen this form growing vigorously without 



