COKEESPONDENCE, 287 



The Microscopic Structure of Ancient and Modern Volcanic 

 Rocks is the title of a most valuable paper read before the meeting 

 of the Geological Society, on Nov. 4th, by Mr. J. Clifton Ward, F.G.S. 

 Uufortunately we have not space for a sufficiently long abstract in 

 the present number, but we nevertheless call the attention of our 

 readers to the subject. We shall give a full account of it in our next 

 number. 



Eow to Make exceedingly Thin Glass Covers. — The following 

 exceedingly interesting paper we quote in full from the ' Quarterly 

 Journal of the Quekett Club,' Oct. Mr. G. J. Burch, who is the 

 author, says: — "Take a piece of glass tube of about ^ inch bore, seal 

 up the end with the blow-j)ipe, and continue the heat until the glass 

 is so soft tbat it will fall out of shape, unless you keep turning it 

 round ; remove it from the flame, and blow into it with all your 

 strength. It will be seen to swell, at first slowly, and then suddenly 

 to a large bubble of very thin glass. Supposing the tube to have 

 been sealed up with as little glass as possible, it may be blown 

 out to about 4 inches diameter. When cold, break it up, and cut 

 the pieces to shape with a ' writing diamond.' The glass in this 

 state is of course convexo-concave ; practically this is of little con- 

 sequence unless the objects are to be mounted dry, when it is liable 

 to be broken. In order to flatten it, place a piece of the thin glass on 

 a perfectly flat piece of platinum foil, and dejiress it for a moment into 

 the Bunsen flame ; as soon as it is red hot, it will sink down to the 

 flat foil. This also has the eifect of annealing it. On measuring a 

 piece of this glass with the micrometer, I found it to be = 270 o i^^ch 

 = • 0004 inch. In the ' Monthly Microscopical Journal,' vol. viii., 

 page 270, Dr. Eoyston-Pigott says : — ' The thinnest glass in my 

 possession measures 2^ thousandths.' Now 2^ thousandths = -0022, 

 and : ^^|f =5-5. So that his thinnest glass is 6^ times the thickness 

 of mine." 



CORKESPONDENCE. 



EOSS AND CO.'S iTU AND BeNECHE's No. 7. 

 7'o the Editor of the ' Monthly Microscopical Journal.' 



MiNSTEE Court, York, November 7, 1874. 

 SiRj — Until of late years England stood unquestionably at the head 

 of all other nations in the production of object-glasses for the micro- 

 scope, but now Paris, Vienna, Berlin, Munich, and Boston dispute the 

 palm with London ; and there is no denying that they are very 

 formidable rivals. 



There are, of course, great difficulties in the way of estimating 

 the relative merits of objectives so long as they are handled by 



