CHAP. XII.] " ELECTRICITY AND MAGNETISM. 383 



as therein is excepted, that is in page 6 ? Why does not 

 this determine them ? to wit 



/dz\ 2 /dz\ 2 f 2 max. for a shed. 



( - i -f- 1 ) maximum, , . 



\dx/ \dy/ \ z nun. for a course. 



Or if this does determine them, how does it resolve itself 

 into "first finding," etc.? 



I am glad you like Strutt on sky-blue. You see he 

 sees his way now to a new theory of double refraction. 

 Looking at your old letter again, I don't quite see the force 

 of either of your objections to space of more than three 

 dimensions. First, you ask if we can think some of the 

 dimensions and not others, then which ? Surely one might 

 answer, that depends depends namely on your circum- 

 stances on circumstances which in your circumstances you 

 cannot expect to judge of. 



" I can easily believe," as Darwin would say, that before 

 we were tidal ascidians we were a slimy sheet of cells float- 

 ing on the surface of the sea. Well, in those days, the 

 missing dimension, and the two forthcoming ones respect- 

 ively, kept changing with the rotation of the earth, we now 

 know how, but could not guess then. So, now, the missing 

 dimension or dimensions, if any, might be determined by 

 circumstances which we could not tell unless we knew all 

 about the said dimension or dimensions. 



To PROFESSOR LEWIS CAMPBELL. 



Glenlair, Dalbeattie, 19th October 1872. 



. . . Lectures begin 24th. Laboratory rising, I hear, but 

 I have no place to erect my chair, but move about like the 

 cuckoo, depositing my notions in the chemical lecture-room 

 1st term; in the Botanical in Lent, and in Comparative 

 Anatomy in Easter. 



I am continually engaged in stirring up the Clarendon 

 Press, but they have been tolerably regular for two months. 

 I find nine sheets in thirteen weeks is their average. Tait 

 gives me great help in detecting absurdities. I am getting 

 converted to Quaternions, and have put some in my book, in 



