244 MEMORANDA. 



the liquid and lay them on a piece of calico spread out on a 

 table or other fiat surface. 1 then remove most of the liquid 

 from one of the pieces by rubbing it on the extended calico. 

 Having removed most of the moisture in this way, I place the 

 cover on a piece of buff about 10 inches long and 2 inches 

 wide, fastened on a Jlat piece of wood of the same size, and 

 by means of an old cambric handkerchief or bit of leather 

 twisted round my fore-finger I rub it towards the other end, 

 turning it in its course, when it will generally be found to be 

 quite clean and fit to be put away ready for use at any time. 



With a little practice this method will be found to be a 

 very easy one, and attended with very little risk of breakage. 

 — William Hodgson, 62, York Street, Lambeth. 



ITIetallic impressions of ITIicroscopic Objects. — The transparency 



of some microscopic objects frequently renders it a matter of 

 difficulty to determine satisfactorily the details of their surface 

 structure, or whether indicated lines, dots, or markings, are 

 really dependent upon exterior configuration. Many of these 

 objects, from their translucency, refract and reflect light, in 

 such various directions, that their superficial formation becomes 

 almost a matter of conjecture ; neither is this doubt always 

 to be resolved by viewing them as opaque objects, for in this 

 case also, the same transparency prevents them from intercept- 

 ing and dispersing a sufficiency of light, to render the question 

 a conclusive one. 



The siliceous valves of the Diatomacea are a class of 

 objects peculiarly possessed of the above characteristics. It 

 has long been a point of dispute, whether the markings 

 which nearly all these objects display are invariably caused 

 by projections on their surfaces, or by the mechanism of 

 their internal structure. I have long been of the former 

 opinion. A careful study of the coarser varieties will 

 distinctly prove that the markings are raised ribs or promi- 

 nences on the surfaces ; in some instances occupying one 

 side of the scale only, as seen in the Campylodiscus spiralis, 

 and others. Though the microscope proves this fact satis- 

 factorily in the large species, it fails to do so in the most 

 difficult specimens, chiefly on account of the above-named 

 deceptive appearances, arising from the irregular refraction 

 and reflection of light. • 



It occurred to me that it might be possible to obtain a 

 perfect cast or impression of the structure, and by viewing 

 this as an opaque object, the errors of refraction would be 

 avoided, and a discovery might be the reward of the experi- 

 ment. I have succeeded in effecting this, by means of the 



