442 Note on a new form oj light Plane Mirrors. 



To ensure this requires considerable care. The film after being washed 

 should be dried as quickly as may be in a warm drying chamber, and 

 it is advisable not even to touch the ring with the hand until its sides 

 are quite dry. The dish also in which the silvering solution is placed 

 must be carefully washed just before use, first with nitric acid, then 

 with water, and finally with distilled water ; but on no account should 

 it be wiped or touched with the hand. 



Before taking these precautions, I never succeeded in getting even 

 a tolerable silver surface, though I tried many other devices which 

 need not now be described. Even the best results I have hitherto 

 obtained are not quite satisfactory, so very ready are the films to lay 

 hold of any kind of scum with which they may be brought in contact. 



The best material for the mirror rings I found to be glass, which is 

 much more easily ground to a true surface than most of the metals I 

 tried, and suitable rings of very thin glass may be easily obtained by 

 cutting up beakers or test-tubes with a glazier's diamond in a lathe. 



The edges may then be quickly ground to a plane on a flat lap with 

 coarse emery or carborundum (which I prefer), and finished with fine 

 emery, though as the edges need not be polished it is not necessary 

 to use nearly such fine emery as is required for ordinary optical 

 surfaces. 



The most important point in getting the edge of the ring to a true 

 plane, is to use a very light pressure in grinding, and the thinner and 

 weaker the ring the lighter the pressure required. 



When the grinding is well done, the definition given by reflection 

 from the stretched film is quite equal to that of a worked glass surface 

 of the same area, at any rate up to 2 inches or 2-J inches diameter, 

 and up to this diameter also it makes no preemptible difference 

 to a reflected pencil of rays whether the plane of the mirror is vertical 

 or horizontal, showing that the weight of the film has no appreciable 

 effect in determining its form. 



The weight of these mirrors is practically the weight of their support- 

 ing rings, and with reasonable care a 2-inch ring may be made 

 weighing considerably less than 10 grains, and although it will prob- 

 ably be impossible to rival with them the brilliancy of silvered glass, 

 there are I should think many cases where good definition, and extreme 

 lightness are requisite (as for instance in the suspended mirrors of 

 photographic recording instruments) in which such mirrors as these 

 might be useful. 



