252 Profs. H. Rubens and R. W. Wood 



on 



reduced the deflexion to only 20 mm. To determine the 

 wave-length of the isolated radiation and the energy distri- 

 bution, we employed the same quartz interferometer which 

 had been used in the investigation of the residual rays from 

 potassium, iodide and bromide, a description of which will 

 be found in the paper already referred to. The plates of 

 the instrument J were placed close to the diaphragm F, 

 and the distance between them varied by turning the screw of 

 the interferometer. The micro-radiometer showed periodic 

 increase and decrease of energy as the thickness of the air- 

 film between the plates was increased. 



If we draw a curve representing the observed deflexions 

 as a function of the corresponding thicknesses of the air-film, 

 we can compute the mean wave-length of the radiation from 

 the position of the maxima and minima, and from the 

 " damping " obtain evidence of the spectrum range with 

 which we are dealing. 



It was found expedient to use quaitz plates of much 

 greater thickness than those used in the former investigation. 

 Having better surfaces and being less easily bent, the 

 opposed faces could be brought into much closer proximity 

 than those previously used. The following investigations 

 were made with pairs of plane parallel plates of 2 mm. and 

 7*3 mm. respectively. 



It was found that the readings of the graduated wheel of 

 the interferometer did not give very reliable indications of 

 the thickness of the air-film, especially when the plates were 

 in close proximity. The distance between the plates was 

 accordingly determined in every case, by observing the 

 interference fringes formed by reflecting the light of a 

 sodium flame normally, from the quartz plates. At the 

 beginning of each series of observations, the quartz plates 

 were brought as nearly as possible into contact, by observing 

 the black spot in the interference fringes obtained with 

 white light. The black spot only appeared when the plates 

 were pressed together with the fingers, the distance between 

 them at the beginning of the series being determined by 

 counting the number of fringes which passed a given point 

 as the pressure was relieved. This usually amounted to 

 7 or 8 fringes only, or a very small fraction of the wave- 

 length under investigation. The deflexion of the micro- 

 radiometer was now taken, and the plates slowly separated by 

 turning the screw, until 20 more fringes had crossed the 

 mark. This corresponded to an increase in thickness of 

 10Xd = 5"89/x. The energy was again measured, and the 

 plates again separated by ten sodium wave-lengtl s, as long- 

 as the maxima and minima could be followed. 



