TRANSACTIONS OF SECTION A. 585 



in order to reconcile the extinction of the third spectrum with the simultaneous dis- 

 appearance of the spectra of even orders. To get rid of the distant and fainter 5th, 

 7th, &c., spectra consistently with the other conditions of the problem in general 

 requires a further modification of the main lines, a modification of the appendages 

 already existing, and the introduction of other still fainter appendages. It should 

 he mentioned that it is possible to make the adjustments such that the appendage 

 lines, or some of them, shall overlie the main lines, and become merged in them, so 

 as not in the observations to be distinguishable from them. 



Having, by the geometrical treatment of some simple cases, made ourselves 

 acquainted with the general progress of events, it is best to study by obser- 

 vation what the details become with each of the innumerable distributions of light 

 that can be made to pass the coUimating lens, and to record the principal results; 

 for it would be an endless task to attempt the prediction of every phase of so 

 Protean a phenomenon. The observations are of a simple kind, since the image 

 which is formed in each case may be directly viewed in the telescope, and we can 

 also easily ascertain what has been the light that has produced this image, by 

 simply taking out the eye-piece and then looking down the tube of the telescope 

 and through its object-glass. We thus see the coUimating lens, and those parts 

 of the central band and diffraction fringes formed by the slit which have reached 

 the coUimating lens, so that these become known. 



15. On the Luminosity observed tvhen a Vacuum Bulb is broken. 



By John Burke. 



It was noticed by Beccaria that a luminous effect was produced when vacuum 

 bulbs were broken in the dark. After making some experiments upon the subject 

 he attributed the luminous appearance, not to the breaking of the glass, but to the 

 dashing of the external air, on the inside, when it was broken. Professor J. J. 

 Thomson, in his work on ' Recent Researches in Electricity and Magnetism,' has 

 brought forward the phenomenon observed by Beccaria as likely to confirm 

 Mr. Crookes's theory of phosphorescence in a vacuum tube. The two phenomena, 

 however, that of phosphorescence in a vacuum tube and that produced by the 

 breaking of vacuum bulbs, seem to be totally distinct ; and the luminosity in the 

 latter case seems to be due to the collisions of the particles of glass with each 

 other. Beccaria also obtained a light when air was allowed to strike against 

 bodies placed inside a receiver, by the bursting of a bladder, but this was un- 

 doubtedly due to the burning portions of the bladder, that were stopped in their 

 descent by the articles within the receiver, which he appears not to have observed. 



The following experiments tend to disprove Beccaria's theory. A number of 

 fragments of broken glass were supported at the mouth of the receiver of an air- 

 pump by an arrangement such that when air was allowed to rush into the 

 receiver by the removal of a thick glass plate which served to cover the opening 

 at the top, the pieces of glass were capable of descending with the air. The same 

 luminous effect was obtained as when vacuum bulbs were broken. 



A single spark was visible when only one piece of glass was employed, and 

 appeared as though it had been caused by the striking of this against the side of 

 the receiver. When no fragments of glass were employed no light was observed, 

 even when a number of articles were placed at the bottom of the receiver. 



Air was made to issue forth, on the surface of various substances, and especially 

 on the sharp edges of broken glass, from bottles containing the air in a highly 

 compressed state, but without any luminous effect. 



The same results were obtained with other gases besides air. 



A light was obtained when two large pieces of glass were violently struck, 

 but no light was observed by the mere breaking of glass either in air or in a vacuum. 



Other substances instead of glass were also employed, such as cast iron, steel, 

 copper, ebonite, sealing-wax, hone, but without any luminous effect. 



All the experiments made seem to point to the conclusion that the phenomenon 

 is not due to the violent impact of the air on the glass but to the collisions of the 

 fragments of glass with each other. 



