10 THE CHEAPEST FORM OF LIGHT. 



with a circular diaphragm covered with ground glass; a lens in front of • 

 the slit casts on one portion of it an image of the white circle formed by 

 the ground glass, which image is the same size as the illuminating organ 

 of the insect and forms a spectrum of the same height in the reserved 

 portion of the field. A suitable disposition of lenses ])laced between the 

 glass screen and the siderostat enables any degree of illumination to be 

 given to the former, from full sunligbt to nearly absolute darkness. If 

 the normal spectrum be studied, a grating is selected of such open ruling 

 that the entire visible spectrum of the first order can be seen in the field, 

 but the grating is first so placed that what is seen is not the spectra but 

 the reflected image of the slit, the grating thus acting (at first) the part 

 of a mirror; so that the observer first sees the two circles of light of ap- 

 proximately equal size and brilliancy, one formed by the insect, the 

 other by the sunlight, and the light of this latter, by the arrangement 

 of lenses between the screen and the siderostat is then adjusted so that 

 while remaining of the size of the insect, it is judged to have the same 

 intrinsic brilliancy, or, at any rate, not a superior one. 



The essential thing is that a photometric comparison shall be made 

 of the two lights before the spectra are formed, and that under these 

 conditions the sunlight is equal but not superior to that of the insect. 



The necessary condition of equality of the two lights from which the 

 spectra are to be formed, having thus been secured, the grating is moved 

 until the two spectra are brought into the field. The result of this 

 direct test is that the solar spectrum when intrinsically of the same 

 brightness, or even when clearly of less brightness than that of the in- 

 sect, extends somewhat further toward the red and distinctly further 

 toward the violet, the insect light being more intense than that of the 

 sun for equal lights, in the green, but ending more abruptly on the 

 violet side. 



It may be added that when the insect's light grew brighter the incre- 

 ment appeared to be more in the blue end or as if the average wave- 

 length diminished with the intensity, but there was not opportunity to 

 put this beyond doubt. 



Photometric observations in the prismatic spectrum were made pre- 

 viously to the adoption of the arrangement above detailed, the first 

 being on July 1, 1889, using thoracic light. The insect was mounted on 

 an adjustable stand, to which it was attached loosel}'', so as to give it 

 such freedom of motion as is needed to insure its emitting the light. 

 It was consequently necessary to readjust its position incessantly, and 

 this necessit}^ constitutes a very obvious difficulty. The thoracic light 

 spots are two ovals, each about 2""" by 1"5""" (see plate I, Fig. 1). 

 Their light is not so bright as the abdominal light, but much steadier? 

 and like that, of a decidedly greenish hue. One of these oval spots was 

 placed over the center of a slit, open just enough to receive the light, or 

 about 1*5™". This slit was in the focus of a glass lens of 8°" aperture 



