AND MODERN PHYSICS. 25 



crystals of saltpetre, which I have been cutting up 

 into plates to-day in hopes to see rings." 



In July, 1849, he writes * : , 



"I have set up the machine for showing the rings in 

 crystals, which I planned during your visit last year. It 

 answers very well. I also made some experiments on com- 

 pressed jellies in illustration of my props, on that subject. 

 The principal one was this : The jelly is jKHired while hot 

 into the annular space contained lictwecn a paper cylinder and 

 a cork ; then, when cold, the cork is twisted round and the 

 jelly exiKvscd to polarised light, when a transverse cross, x, 

 not -f, apjK'ars, with rings jis the inverse square of the radius 

 all which is fully verified. Hip ! etc. 1J.K.D." 



And again on March 22nd, 1850: 



"At Practical Mechanics 1 have leen turning Devils of 

 sorts. For private studies I have been reading Young's 

 'Lecture*/ Willis's 'Principles of Mechanism/ Moseley's 

 ' Engineering and Mechanics, 1 Dixon on * Heat/ and Moigno's 

 'Kejxjrtoire d'Optique.' This last is a very complete analysis 

 of all that has been done in the optical way from Fresnel to 

 the end of 1841), and there is another volume a coming which 

 will complete the work. There is in it, besides common optics, 

 all about the other things which accompany light, as heat, 

 chemical action, photographic rays, action on vegetables, etc. 



" My notions are rather few, its I do not rut*rt<iin them 

 just now. I have a notion for the torsion of wires and rods, 

 not to be made till the vacation ; of experiments on the action 

 of compression on glass, jelly, etc., numerically done up; of 

 pajxjrs for the Physico-Mathematical Society (which is to 

 revive in earnest next session!); on the relations of optical 

 and mechanical constants, their desirableness, etc. ; ami su>- 

 lension bridges, and catenaries, and elastic curves. Alex. 

 Campbell, Agnew, and I are ap]X)inted to read up the subject 

 of i>eriodical shooting stars, and to prepare a list of the 

 phenomena to l>e observed on the Oth August and 13th 

 " Life of J. C. Maxwell," pp. 123-129. 



