SIR. WILLIAM HUGGINS CAMPBELL. 309 



sent into the instrument, together with the star's light, and so form a spectrum 

 by the side of the stellar spectrum for convenient comparison with it. 



This was not all. As the telescopic image of a star is a point, its spectrum 

 will be a narrow line of light without appreciable breadth. Now, for the 

 observation either of dark or of bright lines across the spectrum a certain 

 breadth is absolutely needful. To get breadth, the point-like image of the star 

 must be broadened out. 



As light is of first importance, it was desirable to broaden the star's image 

 only in the one direction necessary to give breadth to the spectrum ; or, in 

 other words, to convert the stellar point into a short line of light. Such an 

 enlargement in one direction only could be given by the device, first employed 

 by Fraunhofer himself, of a lens convex or concave in one direction only, and 

 flat, and so having no action on the light in a direction at right angles to the 

 former one. * * * 



It is scarcely possible at the present day, when all these points are as familiar 

 as household words, for any astronomer to realize the large amount of time and 

 labor which had to be devoted to the successful construction of the first star 

 spectroscope. Especially was it difficult to provide for the satisfactory intro- 

 duction of the light for the comparison spectrum. We soon found, to our dis- 

 may, how easily the comparison lines might become instrumen tally shifted, 

 and so be no longer strictly fiducial. As a test we used the solar lines as 

 reflected to us from the moon — a test of more than sufiicient delicacy with the 

 resolving power at our command. 



Then it was that an astronomical observatory began, for the first time, to 

 take on the appearance of a laboratory. Primary batteries, giving forth noxious 

 gases, were arranged outside one of the windows; a large induction coil stood 

 mounted on a stand on wheels so as to follow the positions of the eye end of 

 the telescope, together with a battery of several Leyden jars; shelves with 

 Bunsen burners, vacuum tubes, and bottles of chemicals, especially of speci- 

 mens of pure metals, lined its walls. 



In 1870 my observatory was enlarged from a dome of 12 feet in diameter to 

 a drum having a diameter of IS feet. This alteration had been made for the 

 reception of a larger telescope made by Sir Howard Grubb, at the expense of a 

 legacy to the Royal Society, and which was placed in my hands on loan by 

 that society. This instrument was furnished with two telescopes, an achro- 

 matic of 15 inches aperture and a Cassegrain of 18 inches aperture, with mir- 

 rors of speculum metal. At this time one only of these telescopes could be in use 

 at a time. Later on, in 1882, by a device which occurred to me of giving each 

 telescope an independent declination axis, the one working within the other, 

 both telescopes could remain together on the equatorial mounting, and be 

 equally ready for use. 



* * * It is not easy for men of the present generation, familiar with the 

 knowledge which the new methods of research of which I am about to speak 

 have revealed to us, to put themselves back a generation, into the position of 

 the scientific thought which existed on these subjects in thie early years of the 

 Queen's reign. At that time any knowledge of the chemical nature and of the 

 physics of the heavenly bodies was regarded as not only impossible of attain- 

 ment by any method of direct observation, but as, indeed, lying altogether outside 

 the limitations imposed upon man by his senses, and by the fixity of his position 

 upon the earth. 



It could never be, it was confidently thought, more than a matter of pre- 

 sumption, whether even the matter of the sun, and much less that of the stars, 

 were of the same nature as that of the earth, and the unceasing energy 



