146 COLOR-BLINDNESS IN ITS RELATION TO 



is struck by it; the other circumstance is the manner in which the 

 knowledge of the method, as well as the principles for the introduction 

 of an official control, has been spread throughout our country. This 

 publicity was not reached in the usual way, through the medium of 

 books, but by oral exposition, practical application, and de visu. Our 

 idea is confirmed by the fact that this personal influence is recognized 

 in the cases brought to our knowledge in England and France, where 

 the scrutiny and control over the sense of color amongst the personnel 

 of railways has been introduced, particularly on the Great Northern 

 Eailway of the first-named country, and the Paris Lyon s-Mediterra- 

 nean line. But even without this support, our experience would suffi- 

 ciently convince us that it is important, if not necessary, to explain the 

 question orally and de visu to the physicians, and especially to railway 

 employes. Speech, in such cases, has an undoubted advantage, and 

 our experience of the last months of 1876, when the Amphitheater of 

 Physiology at CJpsal was often the resort of physicians, railway officials, 

 and individuals afflicted with color-blindness, proves to us that this 

 kind of communication, especially in such a case as that now before us, 

 undoubtedly exerts a more powerful influence than the best written book. 



The warm interest with which the question has been followed up by 

 the journals is too important a circumstance to be forgotten. Not only 

 has this kept the public on the alert, but the movement begun by us 

 has been continued in other countries. Immediate efl'ects were visi- 

 ble amongst our neighbors. In Finland, Dr. L. Krohn, who acquainted 

 Tiiraself by correspondence with the method and principles used in 

 Sweden, has already examined the railway officials of his country. A 

 locomotive and ambulance were placed at his disposal for examining 

 the personnel the whole length of the line. All was accomplished in 

 twelve days. He discovered amongst twelve hundred individuals exam- 

 ined, sixty color-blind, that is, 5 per cent. They were distributed as 

 follows: four red -blind; twenty-five green-blind; and thirty-one incom- 

 Xjletely blind. 



We have as yet heard of no results from our method in Denmark and 

 Norway, but the qnr>stion is there under serious examination, as the 

 Physiological Insti'ite of Upsal has been visited on different occasions 

 by two Danish pli . sicians, Dr. Slidelin and Dr. Fontenay, sent by the 

 railway manager? )f Zeelaud, and by Mr. Hagen, assistant at the Physi- 

 ological Laborat ry of Cbristiania, sent by the Medical Board of Norway 

 at the suggest jn of Professor Worm Miiller. I have thus had the 

 l^leasure of exi laining to them the method of examination as well as the 

 practical prir. ;iples and rules I propose to communicate in this work. 



II.— THE NATUEE OF COLOK-BLINDNESS. 



It is not my intention to treat this question in a thorough manner, 

 but simply to mention what is essential to the practical end in view. 

 It has been known for a long time that every one does not possess 



