268 Messrs. Langley and Very on the 



a part of which arises from the fact that its light is not only 

 fitful but unequal, being of very varying intensities when not 

 wholly intermittent. 



The simplest way in which the experiment can be per- 

 formed is perhaps the following : — 



The insect is placed immediately in front of the slit of a 

 spectroscope so that the light of its thoracic or abdominal 

 portion falls upon the slit. This forms a narrow spectrum 

 which should be brought into the lower or upper half of the 

 field, the insect being attached to the spectroscopic apparatus 

 in a position as nearly fixed as possible. The spectroscope is 

 now placed with the axis of its collimator in the line of a ray 

 of sunlight cast from a heliostat without. In the path of this 

 ray is a screen 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 por- 

 tion of the field. A suitable disposition of lenses placed 

 between the glass screen and the siderostat enables any degree 

 of illumination to be given to the former, from full sunlight 

 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 approximately 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 spec- 

 trum when intrinsically of the same brightness, or even when 

 clearly of less brightness than that of the insect, extends some- 

 what further toward the red and distinctly further toward the 

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



