444 A SHORT HISTORY OF SCIENCE 



PHOTOGRAPHY. Scheele, the Swedish chemist, appears to have 

 been the first to study the effect of sunlight on silver chloride. Others, 

 including Rumford and Davy, observed the chemical properties of 

 light, but it was Wedgwood who, in 1802, made the first photograph 

 by throwing shadows upon white paper moistened with nitrate of 

 silver. Wedgwood was unable, however, to fix his prints. 



Daguerreotypes, taken on silver plated copper, date from 1839, 

 and were made by covering the copper with a thin film of silver iodide, 

 a compound sensitive to light. The image was developed by mer- 

 cury vapor and fixed by sodium hyposulphite. The discovery of the 

 fixing power of hyposulphite was in itself alone of immense impor- 

 tance. With the name of Daguerre, who began experimenting in 1826, 

 that of a fellow countryman and partner, Niepce, is intimately asso- 

 ciated. 



The subsequent development of photography is due to a host of 

 workers. The collodion film which underlies all modern work was 

 first introduced in 1850. It is said to be a practically perfect medium 

 because totally unaffected by silver nitrate. 



ANAESTHESIA. THE OPHTHALMOSCOPE. Anaesthesia, or insen- 

 sibility to pain, during dental surgical operations was introduced, if 

 not discovered, by Wells, a dentist of Hartford, Connecticut, who 

 himself took nitrous oxide gas for anaesthesia in 1844. The first 

 public demonstration of surgical anaesthesia under ether was made 

 by a dentist, Morton, and a surgeon, Jackson, at the Massachusetts 

 General Hospital in Boston in 1846. Anaesthesia by chloroform was 

 introduced by Simpson of Edinburgh, in 1847. 



The ophthalmoscope, an instrument for examination of the inte- 

 rior of the eye, of inestimable value to medicine, was invented by 

 Helmholtz in 1851. It is said that when von Graefe, an eminent 

 ophthalmologist, first saw with it the interior of the eye he cried out, 

 "Helmholtz has unfolded to us a new world." 



INDIA-RUBBER, the coagulated and dried juice of the rubber 

 tree, first reported by Herrera, " who in the second voyage of Columbus 

 observed that the inhabitants of Hayti played a game with balls made 

 'of the gum of a tree' and that the balls although large were lighter, 

 and bounced better, than the windballs of Castile," was at the end of 

 the eighteenth century still a curiosity, employed by Priestley, among 

 others, as an eraser or "rubber." 



Rubber is a hydrocarbon soft when pure but readily hardened by 



