METHODS OF STUDYING VISION IN ANIMALS 79 



Kohl charges a much higher price — 400 marks for the 4" x 5" 

 size; 300 marks for the 3" x 3" size. 



These mirrors with care are almost absolutely permanent; 

 several speculum metal gratings made by Professor Rowland 

 in 1890-95 show no signs of deteriorating. If through neglect 

 the mirrors do become somewhat dull, they can be repolished 

 by using a fine grade of prepared chalk, moistened in distilled 

 water or alcohol. One applies the chalk and polishes with ab- 

 sorbent cotton kept wholly free from dust. If after long disuse 

 the surface of the mirror appears stained, it can still oftentimes 

 be brought into first class condition by the application of dilute 

 C. P. ammonia. This also is applied with absorbent cotton. 

 After using the ammonia the mirrors are cleansed thoroughly 

 with distilled water, and then polished with prepared chalk 

 as before. Finally if through neglect the surface has deteriorated 

 beyond the point where the above simple remedies will restore 

 them, they can always be repolished by Brashear at a fractiou 

 of their original cost. 



Professor Anderson of Johns Hopkins has devised a method 

 whereby mirrors of a permanent kind can be made by depositing 

 various metals possessing high indices of reflection, directly 

 upon plate-glass by an electrolytic process. These mirrors may 

 now be obtained in gold, silver, nickel and platinum. They 

 are far less expensive than speculum mirrors, and make a satis- 

 factory substitute for them when expense has to be considered. 



/. Instruments for measuring wave-length and intensity. — 

 Selenium cell and auxiliary apparatus. — Professor A. H. Pfund 

 has devised for us an exceedingly delicate method of measuring 

 the energy of the various monochromatic bands after being 

 diffused by a ground glass or plaster of par is surface. He gives 

 the following brief description of the method: " In order to 

 measure the energy carried by a beam of monochromatic rays, 

 some bolometric device is usually chosen. When the amount 

 of energy to be measured is so very minute, the sensitiveness 

 of the best bolometer is found insufficient, and recourse must 

 be taken to some other form of energy-measuring device. The 

 wave-length interval over which the present work is to extend 

 lies within the visible spectrum, and since it is precisely in this 

 region that the selenium cell is so very sensitive, it was chosen 

 above others to serve as a radiometric device. 



