36 CORRESPONDENCE. 



which from that time is a dead member. Tracing the cell walls 

 further the colour deepens, and the septa become thicker, till at last 

 the walls split, giving the now rotten cell a detached appearance ; but 

 from the first indication of disease to the final rotten state, no vital 

 activity can be discovered. In all the pliases the starch granules 

 remain unaltered, completely resisting this peculiar decomposition. 

 The disease is evidently conducted throughout the tuber, in the sub- 

 stance of the cell walls. Of its origin I ofter no opinion. If it 

 arises from a deficiency of the vital principle or protoplasm, or a 

 want of stamina (so to term it ), the microscope might discover this by 

 a long series of comparisons, for the presence of protoplasm is neces- 

 sary for the preservation and growth of the cell wall. 



Your obedient servant, 



F. H. Wenham. 



The Merits of the Aplanatic Searcher. 



To the Editor of the '■Monthly Microsco^ncal Journal.^ 



The Hayes, Stroud, GLorcESTEESHiKE, Dec. 12, 1873. 

 Sir, — I must apologize for so soon trespassing again on your 

 valuable space ; and should others have wi'itten to you on the subject 

 of my letter, there will be no occasion whatever for its appearance in 

 your Journal. Now, some of your readers (and perhaps Mr. Mayall 

 amongst them, when he wrote to you in the December number) may not 

 have forgotten an able j)aper in the January number of 1871, read before 

 the Microscopical Society by Mr. Mclntirc, " On the Minute Structiu-e 

 of the Scales of certain Insects," where, after the most careful investi- 

 gation of its merits, and earnest wish to do justice to them, he finally 

 decides against the practical utility of the " Aplanatic Searcher." As 

 the article in question can easily be referred to, it is needless to make 

 any quotations from it here ; but it is somewhat irritating to find a 

 piece of apparatus still held up for admiration, after it has been 

 determined nearly three years ago, and probably up to this time, by 

 hundreds of microscopists, that it lacks just that one special advan- 

 tage as a corrector of the defects of an object-glass so triumphantly 

 claimed for it. A substitute for a high power can intleed be con- 

 jured up, by placing a 1^-inch objective between the eye-piece and a 

 lith, but this can only be procm-ed at the sacrifice of what is so greatly 

 prized in an object-glass — sharpness of definition. There is as much 

 diiference between the results obtained from the one and the other, 

 as there is between the finest line of the graver on copper plate and 

 the blurred mark of a chalk pencil on a piece of paper. 



Improvements, moreover, in the optical part of the microscope, as 

 is often the case in other optical and mechanical arrangements, are 

 gained rather by simplifying structure already complex and intri- 

 cate, than by a contrary j)roceediug. Granting the additional media 

 required, beyond those of a single lens, doublet or triplet, for securing 

 the necessary conditions of a good object-glass, then, surely, the more 



