24:2 On the Spectrum of art Irregular Disturbance. 



optical theory of gratings. For this purpose we imagine the 

 aperture of a telescope to he reduced to a horizontal strip 

 bounded below by a straight edge and above by the curve to 

 be analysed, such as might be provided by a self-registering 

 tide-gauge. Any periodicities in the curve will then exhibit 

 themselves by bright lines in the image of a source of homo- 

 geneous light, corresponding to the usual diffraction spectra 

 of the various orders. An aperture of the kind required may 

 be obtained by holding the edge of a straight lath against 

 the teeth of a hand-saw. When the combination is held 

 square in front of the telescope, we have spectra corresponding 

 to the number of teeth. When the aperture is inclined, not 

 only do the. previously existing spectra open out, but new 

 spectra appear in intermediate positions. These depend upon 

 the fact that the period now involves a sequence of two teeth 

 inasmuch as alternate teeth are bent in opposite directions 

 out of the general plane. 



The theory of diffraction * shows that the method is 

 rigorous when the source of light is a point and when we 

 consider the illumination at those points of the focal plane 

 which lie upon the horizontal axis (parallel to the straight 

 edge of the aperture). 



In order to illustrate the matter further, Mr. Gordon con- 

 structed an aperture (cut from writing-paper) in which the 

 curved boundary f had the equation 



y = sin 2x + sin (3x + 1 tt). 



The complete period was about half an inch and the maximum 

 ordinate about one inch. The aperture was placed in front 

 of a 3-inch telescope provided with a high-power eyepiece. 

 When desired, the plane of the aperture could be considerably 

 sloped so as to bring more periods into action and increase 

 the dispersion. 



The light employed was from a paraffin-lamp i. and it 

 was convenient to limit it by slits. Of these the first was 

 vertical, as in ordinary spectrum work, and it was crossed by 

 another so that at pleasure a linear or a point source could be 

 used. In the latter case the spectrum observed agreed with 

 expectation. Subdued spectra of the first order (corresponding 

 to the complete period) and traces of the fourth and fifth 

 orders were indeed present, as well as the second and third 

 orders alone represented in the aperture-curve. But along, 



* See, for example, " Wave Theory of Light," Encyc. Brit. : ' Scientific- 

 Papers,' iii. pp. 80, 87. Make q—Q. 



f Figured in Thomson and Tait's k Natural Philosophy' § 6'2. 

 % Doubtless a more powerful source would he better. 



