432 L. M. Rutherfurd—sStability of the Collodion film. 
i 
The plates were made and the measures conducted by Mr. 
Chapman, my assistant, who is an accomplished photographer 
and familiar with the use of the micrometer. The plates were 
left clamped upon the stage of the micrometer during the inter- 
val between the measures wet and dry, and both these meas- 
ures were consequently made on the same parts of the screw. 
In every case five bisections of each of the two lines were 
made, and the difference between the means of these readings 
nine measures is Rey. 00017, which is 57,355 of an inch; it 
due to the cooling of the glass plate, by the evaporation whic 
takes place the moment the wet plate is taken from 
holder and exposed to the air under the micrometer. This 
excess of distance (Rev. 00017), would be caused by an merease 
of temperature for the dry glass of about 4° F. 
This consideration reveals a source of error in the use of wet 
plates which I have not hitherto considered, since the same 
objection to the method used by Mr. Paschen, as I ae 
it, is that instead of being confined to an investigation 
a comparison of the actual state of the plate when dry, whan 
ought to have been had all the adjustments, manipulations 4” 
instruments been perfect. 
