PHYSICS: C, BARUS 
57 
fringes merely pass horizontally through the field without appreciably chang- 
ing form. 
It makes little difference whether concave or convex lenses are inserted be- 
tween m and m'j except that the objective of the telescope will have to be 
armed with a convex or a concave lens (of the same strength) respectively, 
to assist in focussing. But here again the most vivid effects are obtained 
with the ocular drawn out of focus. I examined lenses of 1, 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. Many of the forms are 
quite visible to the naked eye. 
If the lens is not symmetrical in form, i.e., for plano-convex meniscus and 
other lenses, the simple figures above discussed become more complicated fand 
the fringes multi-annular. 
4. Index of refraction, irrespective of form. — If a plane-parallel trough con- 
taining a solution of mercury potassic iodide is placed between m and m', 
figure 1, normally to the rays, neither the achromatics nor the spectrum 
fringes (broad slit admissible) are affected. Inclination to the normal posi- 
tion will change the size of fringes only. Hence if a piece of glass is inserted 
into the trough with the rays separated as at 2, 3 and 8, 7 in figure 1, one of 
them (say 8, 7) only passing through the glass, the spectrum fringes will 
change form or vanish except at that part of the spectrum in which the index 
of refraction of the glass and of the solution are identical supposing the dis- 
persion coefficients to be nearly enough the same. It is thus of interest to 
determine to what degree this method can be practically utilized. 
In the case of bodies of regular form, like lenses, spectrum fringes and even 
achromatics will usually appear, when the sharply seen, fine slit images coin- 
cide in the principal focus (i.e., the position of the ocular for parallel rays). 
But the fringes will as a rule be in other focal planes. 
5. The same. Glass plate. — In relation to the principle, §4, in question, 
if a plate of glass of higher refractive index is introduced into the solution 
and traversed by one ray only, the original intensely black, nearly horizontal 
bands in the spectrum are changed to much finer lines, at a considerable 
angle (45°, etc.) to the horizontal. This inclination is symmetrically down 
toward the red, or up toward the red, according as one beam or the other 
traverses the glass. Moreover the size of the fringes now decreases, in much 
more pronounced ratio from red to violet. The achromatics have neces- 
sarily vanished. It would need special compensation (horizontal spectrum 
fringes) to restore them and from this compensation the difference of index 
between solution and glass could be computed. Had there been no difference, 
the horizontal fringes would have been retained in that part of the spectrum. 
Such experiments may be made with astonishing ease and accuracy, and I 
hope soon to communicate quantitative results bearing on this and the preced- 
ing paragraph. 
