SBi" Dr. G. C. \^allicli on the Structure of the Valves of Diatoms. 



botli by direct and oblique llgbt, being placed within a box of 

 sufficient length to exclude the rays coming in other directions, 

 and examined with the naked eye, an opera-glass, and a hand- 

 glass of long focus, at varying distances. The experiment was 

 quite satisfactory so far as the production of the appearances 

 visible under accurate and faulty focusing is concerned, even to 

 the creation of dots where there ought to be pyramidal four- 

 sided figures ; but so identical are the appearances engendered, 

 from whichever side the plates are viewed, that it is almost im- 

 possible to determine, from mere eyesight, which surface is 

 nearest the observer. The same sort of difficulty may be noticed 

 in looking directly through a plano-convex lens held up to the 

 light at some distance from the eye. It is hardly to be wondered 

 at, therefore, that the Diatom- valve should be equally intractable 

 in this respect. 



It is, I presume, quite unnecessary for me to enter into the con- 

 cluding question raised by Professor Schultze with regard to the 

 foraminated structure of certain Diatom-valves, and more especi- 

 ally oiTsthmia and Coscinodiscus, since few,if any, microscopists in 

 this country will be found to coincide with the opinion he ex- 

 presses. That the thinning away of portions of the valvular 

 surface is sometimes extreme admits of no doubt ; but that it 

 normally amounts to actual perforation is negatived by a number 

 of well-known facts and appearances. Under these circumstances 

 I only invite attention to the view he has hazarded, in order to 

 show the danger of reliance on Welcker's test as applied to such 

 organisms as the most delicately sculptured of the Diatoms. 



It only remains for me to express my conviction — one arrived 

 at under no undue bias in favour of lenses of British manufac- 

 ture — that the apparently unavoidable and repeated succession 

 of light and dark points, on which so much stress has been 

 laid by Welcker and Schultze, is indicative of a very limited 

 degree of penetrating power. Of the high qualities of the Hart- 

 nack combination I can speak with confidence from the perfor- 

 mance of one in my possession. It equals any lens I have ever 

 seen for clearness of definition as well as penetrating capacity ; 

 but the necessity for the employment of a drop of fluid between 

 the thin glass cover of the slide and the front combination must 

 always be regarded as an inconvenience and a drawback to its 

 employment, more especially since our first-rate English lenses 

 eflfect all that the Hartnack combination can effect, without any 

 such supplementary aid, and under a much lower degree of mag- 

 nifying power. 



-^ Postscript. — The following is a statement of the experiments 

 conducted with a view to ascertain the relative thicknesses of 



