1893.] MICROSCOPICAL JOURNAL. 109 



which weathering could act. As a consequence the rock has be- 

 come split up into a number of irregularly-shaped aggregates, 

 and as these have the necessary interlocking projections and re- 

 cesses, we again find the same peculiar kind of flexibility as in 

 the Kaliana rock. We have excluded every other possible ex- 

 planation ; the Charli rock contains no mica or other flexible 

 mineral, and the only point they have in common with each 

 other and with other flexible sandstones is that in each case their 

 nature is such that they have weathered into a number of granules 

 separate from each other, and only holding together by the in- 

 terlockingf of their irresfularities. 



LETTERS TO THE EDITOR. 



Note. — Thi^ column is open to all correspondents -who xvrite upoti the 

 topics enumerated under " Problems ^^'' or %vho gife other informatio7t of 

 interest. The fact that aproble7n has been ans%vered once need not deter our 

 friends from making additioial comments. To facilitate referetice, corre- 

 spondents should cite the fiumber as well as the page o/i -which have ap- 

 peared letters a?id queries to which reference is made. The editor is not 

 responsible for the views of others published in this periodical. 



(8) Light for Photographing. — I submit the following answer 

 to Problem No. 5 : Use a short focus (5 or 6 inch) landscape 

 photographic lens. This will produce a good general view of 

 the object, and if the lens is a good one will show many details. 

 — S. G. S. 



(9) Farrant's Medium. — I would say in answer to Problem 

 No. S : Farrant's medium is not worth even 3 cents per oz. if 

 not well filtered and microscopically clean. Did F. ever try to 

 filter the thick, gummy liquid } There is a large sized wrinkle 

 and some tedious manipulation involved in the process, which 

 F. does not dream about. — S. G. S. 



(10) Dry Objectives. — I have been noticing the various let- 

 ters on this subject (page 3o) . I think that the real point, after 

 all, is this : when you are looking over a great number of diatoms 

 at one sitting you can slip them in and out very quickly if you 

 have a dry lens, and save a large amount of time. But if some- 

 thing requires very minute examination, the homogeneous lens 

 is the best to use. No Sig. 



Paris, Alar. 7, i8gj. 



(11) The Van Heurck Microscope. — No one who has 

 ever used this instrument, especially in photomicrography, will 

 hesitate to recommend it. Fortunately, I purchased one recently, 

 and many of my previous difficulties in photomicrography have 

 vanished. Gerard Smith. 



London. March 10. 



