In this work and in the work1 reported at the 1916 session of the American Medical Association, the method of applying heat to the cornea has been one of direct conduction by means of hot metal placed in contact with the cornea. Methods of radiation by holding hot objects near the eye, by convection, through hot air or steam, etc., have been unsatisfactory because of the difficulties in exact control both as to intensity and as to quantity.
The difference between success and failure in applying heat in such grave conditions as serpiginous ulcer of the cornea is measured by only a few Fahrenheit degrees, as will be shown in the following experiments and case reports. Moreover, different materials have different degrees of heat conductivity, just as they do of electric conductivity. For an exact definition, therefore, it is necessary to state the intensity as indicated by the thermometer
SHAHAN WE. FURTHER STUDY OF THE EFFECTS OF HEAT ON THE EYE. JAMA. 1917;LXVIII(26):1969-1976. doi:10.1001/jama.1917.04270060377008