EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 99 



poses and suggests itself particularly with a view to the photography of the 

 interferences, such as may be needed in the next section. 



62. The Nernst burner. In fig. 61 (horizontal projection), N is the usual 

 type of Nernst burner with electromagnetic base, the filament being at a. It 

 is inclosed in the rectangular case A, of blackened tin-plate, judiciously pro- 

 vided with holes for ventilation, so as not to allow the appreciable escape of 

 light from the case, except in front. The front is a micrometer slit 5, in ad- 

 vance of which a short end of flanged tubing c is adjustable, so as to cut off 

 the excess of light spreading up and down. The plate holding the slit 5 may 

 be removed by sliding it up on guides. Similarly the slit 5 and the tube c 

 may be removed by sliding the latter horizontally on this plate. O is the lens 

 of the collimator, aO being (nearly) its principal focal distance. This is found 

 by obtaining the sharp image of the slit, reflected by the opaque mirror N 

 on the jaws of 5. 



FIG. 61. 



In order that the interferences may be sharp, the beam of light falling on 

 the grating must be of slight extent (/i inch) laterally. For this purpose a 

 screen with an appropriately wide vertical slit is placed either at F or on the 

 grating table at b. When this is done two slit images may usually be seen at 

 the mirror N and the brownish one is screened off there. To obtain the soli- 

 tary ellipses, the ruled side g of the grating should face the source of light. 

 As the grating is of ordinary glass plate and therefore wedge-shaped, the top 

 and bottom of the grating should be selected so that the wedge and the thick- 

 ness effect act in concert, to separate the two slit images at N, referred to. 

 In this case the undesirable image may be more easily screened off. 



63. Remarks. It was my expectation that the Nernst filament might 

 itself be used as a slit without further appliances than the screen tube c. But 

 this is not adequately the case, as the filament is a little too thick. Without 

 the slit and the tube only, the ellipses are just suggested. Possibly if the 

 white porcelain surface of the Nernst burner were black instead of white 

 porcelain, clearness would be enhanced. But the ellipses would not be useful 

 for measurement. Without the slit, but with the slotted screen T or b, the 

 ellipses are strong but somewhat washed, so that the fine lines to right and 

 left soon vanish. The rings could actually be used for measurement, for 



