THE AID OF THE ACHROMATIC FRINGES. 



35 



16 



It is a curious fact that although the ellipses can not be produced nor the 

 slit much widened, apparently achromatic fringes do occur in white light for 

 a micrometer placement at P such as should produce centered ellipses. 

 Moreover, as the white slit-image is linear, the achromatic fringes are pref- 

 erably made to run transverse to it. They are then exceedingly brilliant, 

 extending much beyond the slit-image, and they travel up and down it with 

 the motion of either micrometer at P or at n, (A./V) , or with the rotation of 

 the rail (Aa) . As there are but four or six fringes with but one or two strong 

 and brilliant, they make an exceedingly sensitive index for measurement. 

 The occurrence of achromatic fringes may also be detected in the solar spec- 

 trum, as all the Fraunhofer lines (homogeneous light) become helical and 

 broad from the cross-hatching due to the fringes. Here with homogen- 

 eous light the fringes are indefinite in number and follow each other contin- 

 uously, whereas with white light but one or two intense black-white fringes 

 appear. Though the achromatic fringes are by far the most 

 brilliant part of the phenomenon, they rarely occur without 

 streamers. The general appearance is roughly suggested in 

 figure 1 6, where ss is the white slit-image in the telescope and 

 a the achromatic fringes moving up and down 55 when A./V or 

 A change. In the lateral glare of the field, however, fan- 

 shaped or radiating coarse fringes bb are seen, intersected with 

 very fine hairlike fringes cc. Probably there is also an intermediate group. 

 These streamers are very useful to register the approach of the achromatic 

 fringes, which move so rapidly that they are easily lost. 



A few measurements or rather estimates were made to coordinate the values 

 of A./V of the micrometer displacement at n and the corresponding rotation 

 Aa of the rail necessary to annul this displacement. To do this the achro- 

 matic fringes were placed on the cross-hair, or better, on the image of the 

 cross-hair at the slit of the telescope, and both readings were taken. They 

 were then displaced by rotating the rail and restored by moving the microm- 

 eter. To measure the rotation an index was placed at the end of the rail 

 (radius 27 cm.) moving over a millimeter scale observed with a lens. The 

 constants of the triangle, figure 13, were 



6 = 20 cm. d = 62 cm. /3 = 7i.3 * = 35-6 



Corresponding readings were found as follows in two separate adjustments: 



