THE AID OF THE ACHROMATIC FRINGES. 77 



The whole is particularly vivid if the fringes are observed with the ocular 

 drawn well forward, quite out of focus. The same end may be reached by 

 adding a diopter spectacle lens, convex or concave, to the objective. The 

 bearing of this will be seen presently. 



56. Curvilinear compensators. If the rays 8 5 and 2 i in figure 79 are 

 brought to coincidence, it is obvious that a lens, either convex or concave, 

 may be inserted between the mirrors m and m' and normal to the rays, without 

 destroying the interferences, though they must be greatly modified in form. 

 If the lens is symmetrically inserted, the two broad slit-images will be equally 

 wide, so that coincidence is perfect. The fringes so obtained (fig. 81) are 

 usually large, brilliantly colored circles, while in case of imperfect plate they 

 become oval and coarse. The large central disk a is achromatic. To center 

 the fringes the mirror m' may be rotated on a vertical and horizontal axis 

 until the symmetrical circular figure is obtained. Here again the individuality 

 and even the approximate position of the achromatic fringe is retained on 

 passing the broad slit-images through each other; but the sequence of types 

 of fringe is peculiar. As the slit-images separate (see fig. 81) toward the right 

 or the left, as a result of the corresponding rotation of m on a vertical axis, 

 the originally colorless disk a of the circular fringes moves to the right or left, 

 but at the same time becomes very vividly colored (b and c}. The coarse 

 fringes now show considerable resemblance to the coronas of cloudy condensa- 

 tion, in which there is also a colored disk. When the slit images have been 

 markedly separated, the disk vanishes and thinner lines appear, at first as 

 complete circles surrounding the fading disk, but rapidly losing curvature to 

 become vertical. Throughout the whole transformation there has been a 

 grouping of symmetrically concentric colored circles on both sides of the 

 achromatic circle. To state this differently, each originally linear fringe in 

 turn, on expanding (slit-images approaching coincidence), contracts vertically 

 and broadens horizontally into a disk, which retains the color of the fringe 

 out of which it originates. The same result may be obtained by moving the 

 lens inserted between m and m' into both rays, fore and aft (direction 8 5}. 

 Similarly the corresponding sequence between horizontal fringes appears on 

 moving the lens up and down. If the m, m', mirrors are moved bodily fore 

 and aft, however, the circular fringes merely pass horizontally through the 

 field, without appreciably changing form. 



It makes little difference whether concave or convex lenses are introduced 

 between m and m', except that the objective of the telescope will have to be 

 armed with a convex or a concave lens (of corresponding strength), respec- 

 tively, to assist in focussing. But here again the most vivid effects are ob- 

 tained with the ocular drawn out of focus. Sunlight falling on the slit without 

 a condenser gives the best definition. I examined lenses of i, 2, and 3 diopters 

 of focal power, concave and convex. There would be nothing against the 

 treatment of stronger lenses ; only the secondary adjustments become increas- 

 ingly difficult, unless special devices are resorted to. Figure 81 shows the case 



