Ives investigated the question further by the phosphor-photographic method. "In brief it consists of this: Phosphorescence, which is excited in various substances by exposure to short waves (blue, violet or ultra-violet), is destroyed by exposure to longer waves (orange, red, infra-red). Thus, a surface of Balmain's paint or of Sidot blende, excited to phosphorescence and then exposed in a spectrograph, will have areas of reduced brightness wherever long-wave energy has fallen upon it. If this surface is then laid on a photographic plate for a short period, a permanent record is obtained on the plate after development." Preliminary tests showed that the method was applicable in the case of weak light such as the firefly spectrum and also if the light is intermittent like the firefly. With Sidot blend (ZnS) the extinguishing action extends from λ = 0.6µ to λ = 1.5µ. A sheet of deep ruby glass, which cut off all the visible rays of the firefly but allowed infra-red to pass, was placed between the firefly light and a surface of phosphorescent Sidot blend which was exposed to the firefly flashes for three and a half hours. No extinction of phosphorescence occurred, while without the ruby glass, extinction, due to the orange rays of the _visible_ firefly light was noticeable in 20 minutes. There is thus no infra-red of an intensity at all comparable to the visible as far as λ = 1.5µ, the lower limit of the phosphor-photographic method. Coblentz (1912) had examined the transparency of the dry chitinous integument of various fireflies (Fig. 10) in the infra-red and reports it to be fairly transparent down to λ = 2.8µ, opaque between λ = 2.8µ and λ = 3.8µ, transparent again to λ = 6µ, and opaque beyond that. The infra-red could, then, if it were emitted, largely pass through the integument which is similar in absorption properties to complex carbohydrates. Transparency of the integument to the ultra-violet was not studied. [Illustration: FIG. 10.--Transmissivity of the integument of fireflies to infra-red radiation (_after Coblentz._)]