Intelligence and Miscellaneous Articles. 73 



once at each of the planes a, c, emerge through d, the planes of in- 

 cidence and emergence will be parallel, and the incident and emer- 

 gent rays will make equal angles with the edge ac, and therefore with 

 a normal to the faces 5, d. Hence the portion of the incident ray 

 which is reflected from the mirror will proceed in a direction parallel 

 and opposite to that portion of the ray which, after internal re- 

 flexion at a and c, emerges through d. 



In order to ascertain that the construction of such an instrument 

 presented no unforeseen difficulties, I requested Mr. T. E. Butters, 

 of 4, Crescent, Belvedere Road, the well-known maker of sextant- 

 mirrors and artificial horizons, to form the faces a, c on the edges of 

 a piece of plate glass, and then had the face d coated with chemi- 

 cally reduced silver. Upon trial, the emergent light was found to 

 be too bright ; but, after smoking the angle adc in the flame of a 

 candle, in order to reduce the intensity of the light, it became per- 

 fectly easy to make the centre of the image of the sun coincide with 

 the object T seen by direct vision. 



An image of the sun of suitable intensity for pointing might be 

 obtained by attaching to the edge of the mirror a piece of tinted 

 glass, of the form of the corner abed, with the faces b, d parallel to 

 the plane of the mirror. 



XI. Intelligence and Miscellaneous Articles. 



ON THE SPECTRAL RAYS OF THE PLANET SATURN. 

 LETTER FROM FATHER SECCHI TO M. ELIE DE BEAUMONT. 



TN a communication addressed last year to the Academy, I an- 

 -*- nounced that, in the spectrum of the planet Saturn, rays were ob- 

 served different from those which are observed in the terrestrial 

 atmosphere, and which are analogous to those of Jupiter. This 

 year I have prosecuted these researches, and I have been able to con- 

 firm the accuracy of what I then stated, and to take more exact 

 measurements. 



Saturn seen through the spectroscope attached to Merz's tele- 

 scope gives in the red a strong ray almost black, and which, when 

 the air is calm, is perfectly so. This ray is the most easy to recog- 

 nize. On the side of the extreme red the spectrum is weak and 

 badly limited, but it permits a trace of another ray to be seen. 

 Between the red and the yellow there is a pretty well defined band, 

 which by its constitution suggests the band D of the telluro-atmo- 

 spheric spectrum, but whose edges are better defined on the side of 

 the yellow than on the side of the red. Beyond the yellow a trace 

 is seen of the region h of Brewster, which is nebulous in our atmo- 

 sphere. Lastly, bands of the rays E, b, and F of Fraunhofer are 

 distinguished; but they are more difficult to measure, and their 

 position has not yet been sufficiently studied. 



The most interesting part was the red zone. To assure myself 

 of its position relatively to the terrestrial atmospheric bands, inde- 



