128 PROCEEDINGS OF THE AMERICAN ACADEMY 



be cut with scissors into several narrow bands, and preserved in a dry 

 bottle. 



To complete the thermograph, a strip is selected from the stock, laid 

 across the rods at the front of the cylinder, and secured in place with 

 a little soft wax. The wax permits the strip to be shifted, until it is 

 proved by measurement that the junction is exactly midway between 

 the rods, when the strip is permanently fixed by soldering it to the 

 rods. A small concave mirror of glass, silvered upon the first surface, 

 is now fixed to the front end of the cylinder, between the copper rods, 

 with glue or varnish. The mirror should be ^ in. in diameter and of 

 f in. focus. "When in place, the junction in the metal strip should be 

 exactly at its focus. 



The brass tube is carefully blackened within, and is provided with 

 diaphragms, the openings in which are determined by the character of 

 the work for which the instrument is intended. The front end of the 

 brass tube is also furnished with a suitably perforated end-piece of the 

 same material as the cylinder. For some purposes the tube of the 

 instrument requires to be made longer than that of the sam2:)le de- 

 scribed above, and for other purposes it may be dispensed with. For 

 linear measurements, as in spectrum work, it is only necessary to re- 

 place one or more of the diaphragms by slits at right angles to the 

 direction of the strip. The apparatus is particularly successful in 

 dealing with small quantities of heat at a point. When employed for 

 this purpose, the condensing mirror is not used, the junction being 

 merely placed behind a small hole in a triple screen, formed by two 

 plates of metal separated by a thick plate of cork or other non-con- 

 ducting substance. 



If for any particular purpose it should be desired to make use of a 

 strip thinner than can be produced by the methods given above, it can 

 be prepared after the following manner. A flat plate of copper is 

 rubbed with graphite and nickel-plated over one half its surface. It 

 is then dried, rubbed with graphite a second time, and the nickel-coated 

 end dipped in melted paraffine nearly to the junction of the nickel and 

 copper. The remaining half of the copper surface can now be plated 

 with iron from a solution of the citrate, the nickel already deposited 

 being protected by its paraffine coating. When the entire thin film is 

 afterward stripped from the copper backing, one half of it will be nickel, 

 the other half iron, and it can be cut into narrow strips as before. 

 The writer has to learn that there is any advantage to be gained from 

 employing strips made in this manner, and they are certainly difficult 

 of manufacture. In general, thin films of electrically deposited metals 



