a filtered car headlamp to study the brown 

 rat at night. They concluded that "There 

 seems no evidence at all that the brown rats 

 are capable of detecting the rays . " 



The reported biological effects of infra- 

 red radiation thus do not preclude its use as 

 a tool for observation. Confirmation is re- 

 quired for individual species, but there is 

 some basis for such experiments. 



Materials 



The viewing equipment used in the follow- 

 ing experiments was a telescope utilizing an 

 electron -image tube with an associated 

 optical system. The electron -image tube 

 (Zworykin and Morton, 1936) consists of a 

 photosensitive cathode, a fluorescent screen, 

 and an electron optical system to focus upon 

 the screen the electrons from the cathode . 

 Two types of infrared telescopes are shown 

 schematically in figure 2 . 



An electron optical system enjoys a dis- 

 tinct advantage over conventional optics, in 

 that the brightness of the image can be in- 

 creased to a higher level than that possessed 

 by the subject. It is a characteristic of the 

 image tube that the brightness of the repro- 

 duced image varies inversely with the square 

 of the magnification of the tube, within limits 

 of the size of the tube and the angular field of 

 view (Morton and Flory, 1946). Observation 

 with the viewer thus offers an additional ad- 

 vantage relative to information obtained by 

 photographic means alone. Moreover, the 

 brightened image may itseli be photographed. 



The electron-image tube requires a high- 

 voltage power source, and the special power 

 unit used was activated by a 6 -volt battery. 

 The light source used in the experiments was 

 a 6 -volt, 30 -watt spotlight with a reflector 

 and an infrared filter, operated from the same 

 battery . 



Experiments on the Reactions of Fish 

 to Infrared Radiation. 



The primary experiments in the main 

 tank at the Fish and Wildlife laboratory in 

 Seattle are concerned with the electrical 

 guiding of finger ling salmon. These ex- 

 periments are conducted in a darkened 

 laboratory since much of the natural sea - 

 ward migration is at night. The procedure 

 further serves to eliminate light as a 

 possible complicating factor in the experi- 

 ments. Observation with infrared radiation 

 then must not introduce a new complicalrlng 

 factor; reaction to ihe tool would preclude 

 its use as detrimental to the guiding re- 

 search. 



An influence of infrared radiation might 

 be manifest in the behavior of the fish in 

 several ways . The fish might be attracted 

 to the radiation or they might be repelled 

 and seek to avoid it; they might use it to 

 maintain orientation (particularly if their 

 "visible" spectru.m includes the wavelengths 

 used); or they might be momentarily 

 startled by the radiation. 



Avoidanc e 



A light-tight cover was constructed 

 about an existing tank in the laboratory, 

 with an enclosed area alongside tlie tank 

 for an observer. The experimental facil- 

 ities are diagrammed in figure 3. The tank 

 was divided into two compartments by a 

 wire -mesh gate that could be raised and 

 lowered by the observer . The experimental 

 area measured 72 by 80 inches, and con- 

 sisted of two equal compartmerils, each 

 72 by 40 inches. The water depth was 3 

 inches, so that the bottom of the raised gate 

 was submerged. The water was stiLl. An 

 overhead light w/as provided. A circular 

 wire cage, 14 inches in diameter, which 

 could be raised by the observer, restricted 



