THE AID OF THE ACHROMATIC FRINGES. 49 



as may be expected with the equation, since the angle B and T are not 

 easily specified with accuracy. They were 



18 = 71.3 7 = 28.4 6 = 21 cm. 



so that theoretically 



o! = 21(0.947 0.476) = 9-9 



26. Direct interferences without cleavage prism. The next step in ad- 

 vance was made by dispensing with the sharp prisms heretofore used for 

 cleaving the rays issuing from a collimator (or the slit simply) in the endeavor 

 to obtain two rays capable of interference. The assembly of apparatus is 

 shown in figure 26, where S is the slit (to be replaced by a Nernst filament 

 or a tungsten filament), m and n the opaque mirrors, pp' the half-silvered 

 plate. The rays dd', diffracted at 5, pass after reflection into c and c' and 

 may be observed by spectre-telescopes placed either at T or T 1 '. In the first 

 experiments the distance Sp r was about 4 meters and the distance mn 10 cm. 

 The mirrors and pp' were on micrometers with the screws normal to their 

 respective faces. The distance mn must be within the limits of the wedge 

 of light from the slit and is therefore small, unless d is very large. Both 

 pp' and n are on the rotating rail (as above), whereas m is fixed. The appa- 

 ratus was also adjustable for reversed rays by attaching an auxiliary mirror, 

 normal to the rays d' prolonged through n. S being distant, this slit must be 

 long, as otherwise the spectrum band will be a mere horizontal line and 

 the fringes difficult to detect. A doublet of lenses, each about 10 cm. in 

 diameter and of the same focal power (1.60 cm.), but respectively convex 

 and concave and having a combined focal distance of about 5 or 6 meters, 

 is of advantage for focussing a large solar image, i to 2 inches in diameter, 

 on the slit. The Nernst or tungsten filament gives the same advantages at 

 once; but the former is too thick, at least for the initial experiments at 

 shorter distances. 



The fringes are exceedingly difficult to find in spite of the brilliant spectra. 

 It was not until after about three days of searching, in which (besides sunlight) 

 the filaments as well as the methods of direct and of reversed rays were used, 

 that the experiment ultimately succeeded with sunlight. The filaments are 

 much less gracious. To obtain the fringes calls not only for very accurate 

 adjustment for horizontal and vertical spectrum coincidence, but the fringes 

 lie quite sharply in a definite focal plane, usually between that of the slit- 

 image and the principal focal plane; the rays must interpenetrate at the 

 plate and finally path-difference must be nearly annulled. And there are 

 other conditions presently to be stated. After being found they are quite 

 strong elliptic spectrum fringes, but when lost nevertheless difficult to 

 rediscover. The slit may be broadened till the spectrum lines vanish, 

 perhaps to more than a millimeter, before they disappear in a uniform 

 spectrum band. 



The achromatics which coincide in adjustment with horizontal spectrum 

 4 



