CORRESPONDENCE. 281 



of the great variation iu outline and striation in the same species, 

 and this, not only when contrasting sporangial forms, and their im- 

 mediate successors with the parent frustules, but as dependent upon 

 surroundings. Disregarding, then, theit fleeting characteristics, and 

 looking at the subject from a somewhat higher plane, and with broader 

 views, simplifying rather than confusing the study of these interesting 

 organisms, I cannot but consider them all as varieties of Ehrenberg's 

 Navirula rhomboides. 



Most of the group to which the so-called Frustulias belong still 

 retain an investing sheath or cuticle, enveloping the frustule when it 

 is separated from the gelatinous mass, or the tubes, and which becomes 

 dark brown upon burning, and often wrinkled. If the burning is in- 

 sufficient, the colour still remains : as most of the dry preparations are 

 now made by incineration, this may account for the colour noticed by 

 Mr, Hickie. In the closely allied Amphipleurece, the wrinkled sheath 

 gives an appearance of resolvability to some frustules even with very 

 low powers ; and I have often noticed this with CoUetonema vulgare. 



H. L. Smith. 



[Professor H. L. Smith having requested me to revise the proof 

 of the above letter, I take the opportunity of stating that I fully 

 agree with him that the genus FrustuUa should be abolished ; and I 

 am also of opinion that Navicula crassinervis is only a form of N. rhom- 

 boides. I may also state that my friend Mr. Hickie is of the same 

 opinion. — F. Kitton.] 



Mr. Tolles' ^th again ! * 



To the Editor of the ' Monthly Microscopical Journal.' 



Boston, Zfai/ 2, 1S7G. 

 Sir, — We have the authority of Mr. Wenham, found on p. 225 of 

 ' M. M. J.' for May last, thart a certain |th objective did work through 

 a plate of glass • 018" thick, and this is given as the thickest it would 

 penetrate and come to a focus on the under surface thereof. This 

 distance iu glass he has repeatedly said is practically the same as iu 

 balsam. 



* Mr. Tolles has sent us besides the above, another communication on the 

 same subject, which, however, we decline inserting. We must positively refuse 

 publication to any other letters relating to Mr. Tolles' particular apertures, which 

 we are sure our readers are heartily weary of. 



By comparing the figures on the above diagrams with the dimensions given 

 by Mr. Wenham on page 153 of this Journal for March, and at page 184 for 

 April, it will be seen that Mr. Tolles in both of them, in order to make out his 

 case, has taken ditierent dimensions of working diameter and focal distance from 

 those of Mr. Wenliam. This is mere contradiction, imd no argument. 



Mr. Wenham in his article in this Journal for April last, page 185, now stands 

 responsible for the statement that " all measurements for ascertaining large aper- 

 tures have hitherto been erroneous, and far in excess of the true pencil." As this 

 is of importance, we shall be glad to afford space in our pages for a proper dis- 

 cussion as a pure question of science. If it is taken up as a personal one, letters 

 having this tendency will not appear in our pages. 



