104 Langley and Yery — Cheapest Form of Light. 



and not to its feeble intensity. The only conclusive method 

 of determining this would appear to be to balance the light 

 from the insect with that of a definite portion of sunlight by 

 any ordinary photometric device ; and having taken this sun- 

 light as nearly equal as possible to that of the insect, though 

 certainly not greater, to let this determined quantity fall on 

 the slit of a spectroscope at the same time with the light from 

 the insect, two spectra being formed one over the other in the 

 same field and at the same time. 



The actual doing this is not so easy as it might appear, 

 owing to experimental difficulties connected with the insect, 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 performed 

 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 por- 

 tion 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 be- 

 tween 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 



