HELMHOLTZ IN BERLIN 



moderate wind blowing over the surface of water 

 will give rise to waves of about a metre in length (a 

 little more than a yard) ; the same waves, in cloudland, 

 at the border of two layers of air, differing in tempera- 

 ture by 10 C. will have a length of two to five kilo- 

 metres (about 2^- miles). Sea waves may reach a 

 length of five to ten metres (say 12 yards), and 

 as these would correspond to cloud waves of from 

 fifteen to twenty kilometres (say over 10 miles), we 

 can grasp the notion that one of these long cloud 

 waves may, at a particular time, cover all the sky 

 above our head. That these views of Helmholtz on 

 cloud formation are correct, has been supported by 

 many observations since made in balloon voyages. 

 It has always been found that in traversing regions 

 in which there are great plains of cirrhi clouds, a 

 sudden change of temperature is noticed in passing 

 from one layer of air into the other. 



5. On Physical Optics. 



A slight sketch has already been given in Chapter 

 VIII. of Helmholtz's researches in physiological optics, 

 and the whole is narrated in full detail in a new edition 

 of his great work, Handbuch der physiologischen Optik^ 

 published in 1894, the year of his death. In this 

 work the dioptrics of the eye are fully discussed, with 

 a wealth of illustration and mathematical power that 

 is truly astonishing. The laws of refraction from 

 p 



