THE AID OF THE ACHROMATIC FRINGES. 51 



In fact, in view of the longer distance d, the slit could be opened to over 

 a millimeter of breadth before the fringes quite vanished from the spectrum; 

 but on using a somewhat stronger condensing system (concave lens of doublet 

 preceding the convex lens) and consequently more oblique rays, a very fine 

 slit was needed to show the fringes. They are thus more easily found when 

 the rays are more nearly parallel. 



With artificial light, again, I obtained no results, even after long searching. 



Operating with two successive slits at about 9 meters from the interfer- 

 ometer, one of which received the light through the other, I found that two 

 independent sets of fringes very different in size and inclination could be put 

 in the field together. The further investigation eventually showed that the 

 size and inclination of the fringes is essentially dependent on the degree of 

 parallelism of the two slit-images. When the images are parallel, the fringes 

 are of maximum size and vertical. When the images are not quite parallel 

 (they incline in opposite directions when the slit is slightly rotated in its own 

 plane from the vertical), the fringes rapidly grow smaller and rotate. With 

 parallel slit-images the spectrum ellipses are centered in the field; otherwise 

 they are very far out of center. The adjustment for actual (not X-like, 

 coincidence must therefore be made with precision if large fringes are wanted. 



Further work was also done with sunlight to obtain more pronounced 

 achromatics. For this purpose a compensator was inserted to equalize the 

 glass path in the half -silvered plate. Huge spectrum ellipses were obtained 

 in this way and their centers were placed above the telescopic field, so that 

 the fringes seen were large horizontal bars. On removing the spectroscope 

 and placing the slit-images out of focus, brilliant achromatics were in fact 

 obtained, of the concentric hyperbolic type, vividly colored and broad be- 

 tween the apices, and diminishing to hair-lines laterally. With these it 

 was possible to enlarge the slit to at least 3 mm., without destroying the 

 fringes, though they became more vague. It is necessary that the slit-images, 

 when in focus, should be quite parallel, otherwise any broadening of the slit 

 will wipe out the achromatics. It was possible to place a plate of ground 

 glass on the far side of the slit without destroying the fringes, but not on the 

 side towards the interferometer. In other respects the behavior was as 

 described in the case of achromatics in the earlier experiments with a cleav- 

 age prism. 



Finally, the spectrum fringes and the corresponding achromatics were 

 obtained with the light of a Nernst filament, at first by focussing an image 

 of it with a strong condenser lens on the slit. The experiments, however, 

 are very difficult. The spectrum fringes are often weak, out of focus, and 

 extremely sensitive to small disadjustments in the horizontal and vertical 

 coincidence of the slit-images. They require a fine slit. When well produced 

 the achromatics are also obtainable on removing the spectroscope when the 

 spectrum fringes are horizontal bars. The achromatics may also be obtained 

 brilliantly without the condenser lens, but the adjustment must in such a 

 case be made first with sunlight, as the spectrum from the Nernst filament 



