REVERSED AND NON-REVERSED SPECTRA. 155 



density, such as would result, for instance, from rise of temperature or from 

 partial exhaustion. Hence the effect observed, though very definite, would 

 correspond to a temperature effect due to electrical currents traversing the air. 

 One should expect the effect of ionization, if appreciable, to be the reverse of 

 this. With voltages high enough to produce sparks in the tube, the inter- 

 ference figures naturally show violent agitation or quiver. If the displacement 

 in question is one ring and S denotes differences, 5(AA/") = 3oXio~ 6 cm. 



If only temperature changes, one may write, roughly, A N.T = constant, 

 T referring to absolute temperature, whence 



8r = 



40X10 



if results found for a similar tube, above, be taken. 



Thus 5r = 2.2Xicr 7 degrees centigrade is the average temperature incre- 

 ment, for the whole length of the tube. 



When but a single cell was used to energize the coil, no effect could be recog- 

 nized. In case of two cells, moreover, when the plenum of air was replaced 

 by a partial vacuum of i cm. or less, so that an arc was seen, no effect was 

 observable, although the reddish light colored the field of the telescope. 



There are two points of view, however, from which the assumption of a tem- 

 perature effect is not admissible. If the pipe is closed, so that the density of 

 the air contained remains unchanged, there is no difference in the phenomenon. 

 But there should not, for the case of constant density, be any effect, unless the 

 nature of the gas is changed. Again, the effect is instantaneous and not 

 increased on keeping the circuit closed. The simple explanation in terms of 

 temperature made above must therefore be taken with reservation. At all 

 events, the effect of ionization would be small and equivalent to a dilution of 

 the gas of but 



-X2.2Xio~ 7 or about io~ 9 

 273 



of its density, when sparks are about to occur. 



94. Mach's interferences. It is frequently necessary to use the interferom- 

 eter in such a way that but one ray passes in a given direction ; i.e., the rays are 

 not to retrace their path. Interferometers of this kind are treated above, but 

 Mach's design offers advantages, which will be presently pointed out. As a 

 rule, in using these interferometers, the center of the elliptic interference pat- 

 tern is remote and the lines are hair-like and found with great difficulty. These 

 annoyances are overcome when the apparatus is put together as follows : 



In figure 96, L is the vertical sheet of light from a collimator impinging on 

 the strip of plate glass gg, half -silvered on one side, toward or near the ends. 

 The pencil L is thus reflected to the opaque mirror N and transmitted to the 

 opaque mirror M (on a micrometer) , and then reflected to the other end g' of 

 the glass strip gg' . Thereafter, both the pencils, Mg' and Ng f , are available; 



