342 JAMES CLERK MAXWELL. [CHAP. XI. 



disc A hung by a wire D, between two discs B, C, the 

 interval being occupied by air, hydrogen, carbonic acid, 

 etc., the friction of which gradually brings A to rest. In 

 order to calculate the thickdom or viscosity of the gas, I 

 require to solve the problem above mentioned, which is now 

 done, and I have the apparatus now ready to begin. We 

 are also intent on electrical measurements, and are getting up 

 apparatus, and have made sets of wires of alloy of platinum 

 and silver, which are to be sent all abroad as standards of 

 resistance. I have also a paper afloat, with an electro- 

 magnetic theory of light, which, till I am convinced to the 

 contrary, I hold to be great guns. 



Spice l is becoming first-rate : she is the principal patient 

 under the ophthalmoscope, and turns her eyes at command, so 

 as to show the tapetum, the optic nerve, or any required 

 part. Dr. Bowman, the great oculist, came to see the sight, 

 and when we were out of town he came again and brought 

 Bonders of Utrecht with him to visit Spice. 



To H. K. DROOP, Esq. 



Glenlair, Dalbeattie, 19th July 1865. 



There are so many different forms in which Societies 

 may be cast, that I should like very much to hear some- 

 thing of what those who have been thinking about it pro- 

 pose as the plan of it. 



There is the association for publishing each other's pro- 

 ductions ; for delivering lectures for the good of the public 

 and the support of the Society ; for keeping a reading room 

 or club, frequented by men of a particular turn ; for dining 

 together once a month, etc. 



I suppose W 's object is to increase the happiness 



of men in London who cultivate physical sciences, by their 

 meeting together to read papers and discuss them, the pub- 

 lication of these papers being only one, and not the chief 

 end of the Society, which fulfils its main purpose in the act 

 of meeting and enjoying itself. 



1 The Scotch terrier of the period. 



