1893.] MICROSCOJ'ICAL JOURNAL. 109 



which wcathrrin*^ could act. As a conseqiiciice the rock has be- 

 come split up into a iiuinl)er of inej^^uiarlv-shapcd a<^^rc<^ates, 

 aiul as these have the necessar\' iiiteilocUin<^ |ir(jjectioiis and re- 

 cesses, we aij^aiu find the same j)eculiar kind of flexibility as in 

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

 ]ilanation ; the Charli rock contains no mica or other flexible 

 mineral, and the only ])oint thev 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 onlv holding together by the in- 

 terlocking of their irregfularities. 



LETTERS TO THE EDITOR. 



Note. — This column is open to all correspondents -who write upon the 

 topics enttmcrated under " Problems" or rvho give other information of 

 interest. The fact that a problem has been ansivcrcd once need not deter our 

 friends from making additional comments. To facilitate reference, corre- 

 spondents should cite the number as well as the page on -which have ap- 

 peared letters and queries to zvhich reference is made. The editor is not 

 responsible for the viezvs oj others published in this periodical. 



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

 to Problem No. 5 ; L'sc a sliort 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 manv 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 trv 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 20). I think that the real point, after 

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

 at one sitting vou can slip them in and out verv 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. Mar. 7, j8qj. 



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

 ever used this instrimicnt. especiall} in photomicrography, will 

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

 and manv of mv previous difficulties in photomicrography have 

 vanished. Gkkakd S.mith. 



LoNDOx. March 10. 



