Light; a course of experimental optics, chiefly with the lantern . ) thatwe get a most beautiful series of diagonal spectra. This,of course, must follow from considerations already dis-cussed, and the greater distances apart of the red images Melbourne Exhibition of 1880-81. There is therefore some hope thatthese beautiful objects may before long be again accessible to such asadmire them. LIGHT. [chap. than the blue. And all these diagonal spectra are perfectlystraight But if, instead of a second grating, we use aprism, with its refracting edge in the rectangular line, it is notso. We still di
Light; a course of experimental optics, chiefly with the lantern . ) thatwe get a most beautiful series of diagonal spectra. This,of course, must follow from considerations already dis-cussed, and the greater distances apart of the red images Melbourne Exhibition of 1880-81. There is therefore some hope thatthese beautiful objects may before long be again accessible to such asadmire them. LIGHT. [chap. than the blue. And all these diagonal spectra are perfectlystraight But if, instead of a second grating, we use aprism, with its refracting edge in the rectangular line, it is notso. We still disperse not only the central pencil of whitelight, but each spectrum produced by the grating; and, asbefore, the blue portions of the spectra are more refractedthan the red. But the deflection is no longer proportional,but dependent on the special dispersion of the prism ; andhence the refracted spectra now appear as parabolic curves,represented at g, Plate III. no. Measurement of Waves.—It is plain that wehave in the phenomena of interference, various means of. Fic. 109.—Nature of Diffraction. measuring the lengths of the waves which produce any givencolour. For many and obvious reasons such measurementsare easiest taken with monochromatic light; and the simplestcase is that of the light from a slit or point passing througha second slit (§ 106). Let a b be a highly magnified repre-sentation of the second card, and c d of the slit in it. Therays which pass perpendicularly through c d wUl none ofthem be retarded, and therefore produce on the retina or ascreen an ordinary white image—the central white as the card a b stops off the main wave-front (§62)every particle of ether in motion all across the width of the IX.] MEASUREMENT OF WAVES. 193 slit produces new secondary waves spreading right and leftof the perpendicular direction : let us take any given incli-nation, c E, D F. Then drawing c w perpendicular to thecourse of the ray, we see that the waves from d hav
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