REVERSED AND NON-REVERSED SPECTRA. 



127 



identical tubes, alternately exhausted as suggested above. The attempted B 

 measurement is at the limits of the method, as has already been instanced 

 in the discussion of errors in the preceding paragraph, and it is not to be con- 

 cluded that data which happen to agree with Mascart's result from a correct 

 application of the present method. In fact, there is no reason for excluding the 

 exceptional values, and the present results are to be regarded as preliminary. 



73. Two (differential) refraction tubes* In the following experiments two 

 identical iron tubes (138 cm. long, of inch gas-pipe) were installed, one being 

 placed in each of the component beams of light, which subsequently interfered, 

 and the tubes were exhausted alternately. There are apparently three advan- 

 tages in this arrangement. In the first place, the sensitiveness is doubled; in 

 the second, the flexure of glass plate should be the same at each tube, in each 

 experiment, and thus fail to disturb the interference pattern. Furthermore, 

 by using the tubes in parallel (i.e., exhausting both at the same time), any 

 irregularity of flexure effect, etc., should be determinable, as the air in both 

 tubes will be identically circumstanced. Finally, the air being inclosed in a 

 thick metallic envelope at both beams is not subject to incidental disturb- 

 ances. An unexpected difficulty, however, was encountered ; for there is reflec- 

 tion of direct spectra from the eight glass surfaces, and this must be specially 

 met. The direct spectrum is easily eliminated by inclining the grating until 

 the reflected interference spectra are at a different level; but reflections of 

 this spectrum are not so easily dealt with. Fortunately they are weak. Even 

 so, they are very annoying, as they overlap the interference pattern and dull 

 it. They could be eliminated by attaching the glass plates obliquely to the 

 axis of the pipes, but this remedy was not thought of at the outset. 



Figure 92 is a diagram of the disposition of the parts of the apparatus. 

 L is the beam of white sunlight from the collimator limited laterally by the 

 wide slit (i inch) 5. G is the grating, T and T' the two refraction tubes, 

 M (micrometer) and N the opaque mirrors, R the refracted and D the dif- 

 fracted (spectrum) beam of light. C is virtually a four-way stopcock (or two 

 3 -way glass stopcocks) leading respectively to the exhaust pump E and dry 



