274 Sutchina — Absorption and Thickness of thin Films. 



Art. XXV. — Absorption and Thickness of thin Films; by 

 C. C. Hutchins. 



The here recorded observations were made in the course of 

 an attempt to find a window for radiometric apparatus that 

 would effectually close such apparatus against all convective 

 air currents, and at the same time introduce less absorption 

 than rock salt or fluorite. It was argued that since the absorp- 

 tion of a body is an exponential function of its thickness, the 

 body being made very thin, the absorption would nearly van- 

 ish even if the coefficient of absorption were high. 



As is shown below, it is an easy matter to prepare permanent 

 films of comparatively large area having a thickness of one- 

 tenth to one-twentieth of a wave-length of light, and an 

 extremely small absorption even for very long waves. 



Preparation of Films. 



Make a collodion by dissolving gun-cotton in amyl acetate. 

 Instead of gun-cotton I have used shavings from the transpar- 

 ent celluloid handle of a discarded toothbrush, and found the 

 result almost as satisfactory. The collodion should be rather 

 thick at first, then gradually diluted until the proper consist- 

 ency is found by making trial films ; then filter through cotton 

 until it is perfectly pellucid. 



For practice one may bend up a ring, say 5 cm in diameter, at 

 the end of a stout iron wire, soldering it so that it may be 

 smooth and flat ; later, any desired support may be used. 



Fill a large evaporating dish with clean water. Dip the 

 rounded end of a glass rod in the collodion and touch the adher- 

 ent drop to the surface of the water in the middle of the dish. 

 The drop instantly spreads to a large disc on the surface of the 

 water. The film quickly sets, by the evaporation of the solvent, 

 so that at the end of a minute or so it may be removed to the 

 support. Slip the wire ring under the margin of the floating 

 film ; bring the ring into contact with the film at the margin, 

 then very slowly and steadily raise the ring to a vertical posi- 

 tion, at the same time withdrawing it from the water. The 

 portion of the film not on the support gathers to a tail at the 

 lower edge. 



The film is then hung up to dry, and as it does so it will be 

 seen to show the Newton black all over its whole surface, being 

 too thin to reflect much light. A little practice enables one to 

 spread a film upon any desired support. When dry they are 

 very strong and do not break from shock. 



