174 Britain's Heritage of Science 



colours are given only when the light falls on the film at 

 the particular angle under which it was originally produced. 

 The process of Joly, introduced in 1897, is free from this 

 defect ; the principle on which it is based is the same as that 

 subsequently employed with great success by " A. Lumiere 

 et Fils," of Lyons, whose method of working, however, 

 differs materially from that of Joly. 



Photography is looked upon by some as a pleasant 

 pastime, by others as an art. The chemical and physical 

 properties of matter which allow the rays of light to form 

 a latent picture, to be subsequently developed, fixed and 

 printed, are in themselves a fascinating study, and there is 

 no limit to the utility of photography as an aid hi scientific 

 investigations. Here, as elsewhere, science exerts its greatest 

 charm when it forms a connecting link between the ordinary 

 interests of our daily life and the abstract questions which 

 engage the attention of academic philosophers. Thus, 

 nearly all problems of geophysics have both an intensely 

 practical and a deeply theoretical side. The commonplace 

 necessity of defining the boundaries of land leads to the 

 demand for accurate maps, and this, again, opens out 

 investigations on the figure and size of the earth. One 

 question suggests another, until abstruse mathematical pro- 

 blems acquire a special interest owing to their connexion 

 with the history of the world's formation. Similarly, fore- 

 casts of weather that shall be helpful to the farmer demand 

 a study of aero -dynamics, involving mathematical treatment, 

 combined with experimental work of high precision, and 

 the ordinary phenomenon of the tides takes us inevitably to 

 problems demanding the genius of such men as Kelvin and 

 George Darwin. 



The ordinary making of maps is a task belonging to the 

 Government services, and it is to officers in the Army and 

 the officials in charge of the various surveys at home, or in 

 the colonies, that we are mainly indebted for our knowledge 

 of geodesy. Such work, important as it is, often receives 

 insufficient acknowledgment because, being co-operative, the 

 share of each man cannot always be clearly defined. But a 

 few examples may be given. 



Captain Henry Kater (1777-1835), the son of a sugar 



