172 Britain's Heritage of Science 



as a star is receding or approaching. Huggins showed how 

 this principle can be applied to stellar motion, and thus 

 laid the foundation of a branch of astronomy which is 

 continuously growing in importance. Previously Huggins 

 had, in conjunction with W. A. Miller, carefully mapped 

 some star spectra; he also had investigated the spectra of 

 nebulae, and found that some of them consisted of glowing 

 gases. In subsequent researches he found the luminosity of 

 comets' tails to be mainly due to carbon compounds. By 

 patient and painstaking work Huggins further developed 

 the methods of obtaining photographic records of stellar 

 spectra, and the important results obtained formed the 

 starting point for the many distinguished astronomers who 

 have since taken up the work. 



Before leaving the subject of Astronomy reference must 

 be made to a notable advance in the construction of re- 

 fracting telescopes. During the middle of last century, the 

 largest lens made had a diameter of sixteen inches. At the 

 exhibition of 1862, Messrs. Chance, of Birmingham, exhibited 

 glass discs of crown and flint twenty-six inches in diameter, 

 and Mr. Robert Stirling Newall (1812-1889), of Gateshead, 

 induced Messrs. Cook, of York, to construct from these 

 discs an achromatic lens of twenty-five inches. This was 

 successfully accomplished, and the telescope is now doing 

 excellent work in the Astrophysical Observatory of Cam- 

 bridge. Larger instruments have been made since, but the 

 step from sixteen to twenty-five inches is one which deserves 

 a permanent record in the history of the subject. 



Modern astronomy, like other branches of science, depends 

 so much on photography that a brief account of the history 

 of this interesting and fascinating art may be here introduced. 



The darkening action of light on silver chloride was first 

 discovered and investigated by the Swedish chemist Scheele. 

 W. H. Wollaston had observed that the colour of the yellow 

 gum guaiacum was altered by the action of light, and Sir 

 Humphry Davy had noted a similar effect in the case 

 of moist oxide of lead. The first actual photographic print 

 was obtained in 1802 by Thomas Wedgwood (1771-1805), 

 who threw shadows on paper moistened with a solution of 

 silver nitrate, and obtained prints giving the outlines of the 



