POPULAR MISCELLANY 



571 



taken by this process are perfectly clean. 

 The plates, moreover, have the quality of 

 preserving the impressed image for a long 

 time after they have been exposed and be- 

 fore it has been developed. The chief dis- 

 advantage of the process is that the photog- 

 rapher using it has very little power to give 

 local intensity to his picture. Till lately, a 

 red light had to be employed in the devel- 

 oping-room, and this was painful to the op- 

 erator. The difficulty has been obviated by 

 adding iodide of silver to the emulsion in 

 the proportion of one part of iodide to 

 eight of the bromide, when an orange light 

 can be used with impunity, and the shadows 

 are given a wonderful clearness. When a 

 collodion emulsion is adapted to a flexible 

 support and used for the negatives, the op- 

 erator is able to do away with glass and its 

 weight, and may store rolls of sensitive ma- 

 terial in the camera itself. Then, by turn- 

 ing a screw, he may place fresh portions of 

 the band in a condition for exposure. After 

 exposure, paper prepared with this emulsion 

 may be moistened with turpentine, and the 

 film bearing the image, almost free from 

 weight and bulk, may be stripped off. To 

 print from these flimsy negatives, it is only 

 necessary to place them on glass. When a 

 flexible support shall have been introduced 

 for the gelatine emulsion, the negative pro- 

 cesses of photography will be almost per- 

 fect. With the collodion emulsion, we are 

 able to get a physical condition of the bro- 

 mide, in which it will answer to the vibra- 

 tion of the rays of the lowest refrangibility. 

 Photographs of the solar spectrum taken on 

 this salt were shown, corresponding to wave- 

 lengths which lie more than three times as 

 far below the red as the distance of the visi- 

 ble spectrum. This process is being applied 

 to the examination of different colorless 

 bodies, with a view of obtaining spectro- 

 scopic analyses of them. It has been dis- 

 covered that the photographic image is ren- 

 dered undevelopable by the action of oxid- 

 izing agents, and this alike whether it be 

 produced on a collodion or a gelatine film 

 or on paper. The oxidation of the image 

 goes on also in ordinary atmospheric condi- 

 tions, more especially under the influence of 

 light, and in it we have a simple explanation 

 of what is known as solarization. A plati- 

 num process, in which an image is produced 



in platinum-black, is about four times as 

 sensitive as the ordinary silver process, and 

 marks the greatest advance in printing that 

 has been made for many years. Another 

 printing process is based on the reduction, 

 by ferrous oxalate, of bromide of silver 

 which has been exposed to light, and was 

 discovered almost simultaneously by Mr. 

 Willis, of England, and Mr. Carey Lea, of 

 Philadelphia. The prints obtained by this 

 process have great permanence, since no or- 

 ganic compound of silver, the great agent of 

 deterioration, is present in them. Mr. Lea 

 has discovered within the last year that the 

 power of development which is shared by 

 most of the organic salts of ferrous oxide is 

 not limited to them, but is possessed also 

 by many of its inorganic compounds. He 

 gives a brief account of his researches in 

 this direction, and of the properties of the 

 different salts of the oxide, in the " Ameri- 

 can Journal of Science " for June. 



The Regnlation of Visnal Tests. The 



Boston "Herald" published last summer a 

 review of what had been done in the United 

 States during the previous twelve months 

 to secure protection against the dangers 

 arising from color-blindness and visual de- 

 fects in seamen, railroad-men, and other 

 persons occupying positions of trust in 

 which the quality of eyesight is important. 

 Three departments of the national Gov- 

 ernment have adopted regulations on the 

 subject. The War Department has or- 

 dered the examination of recruits by test- 

 cards, to test their power of distinguishing 

 objects at a distance, and with worsteds for 

 their perception of colors. The Treasury 

 Department has made the examination of 

 all pilots, as to their ability to distinguish 

 colors, compulsory, with the provision that 

 " a second visual examination will not be re- 

 quired in any case." The Navy Department 

 has ordered a similar examination for all 

 persons in the navy, or who may hereafter 

 enter it. The House Committee on Naval 

 Affairs has reported favorably on a petition 

 of Dr. B. Joy Jeffries and others for the en- 

 actment of a general law of control for the 

 naval -and merchant service, and for a rep- 

 resentation of the United States in an Jn- 

 ternational Congress to agree upon definite 

 standards of color-tests, and has reported a 



