A Method for Adjusting the Collimator of a Spectroscope. 95 



electricity, and therefore exerts a more powerful attraction 

 upon the positive than the negative. (On the different be- 

 haviours of positive and negative electricities, compare also 

 G. Wiedemann and R. Riihlmann.) 



If these experiments show that the passage of electricity 

 from one particle to another takes place in various ways in 

 different substances, and that, independently of the total tem- 

 perature of the mixture, only certain particles are rendered 

 luminous by the electric spark, we may further have to take 

 into consideration that the passage of electricity from atom to 

 atom is capable of producing oscillatory motions of their aether 

 envelopes, yet without augmenting in corresponding measure 

 the vis viva of the progressive motion of all the particles, as 

 would be necessary according to the theory of gases. We find 

 an analogy to this in the augmentation of the interior motion 

 in a molecule without a corresponding increase of the oscilla- 

 tory motion of the whole when non-luminous discharges pass 

 through different gases : the latter undergo decompositions 

 which otherwise would only be produced by considerable ele- 

 vations of temperature. Similarly, in the phenomena of 

 fluorescence, by the incident aether-vibrations the vis viva of 

 the translatory motions is, for certain vibrations, increased in 

 a way that corresponds to a direct augmentation of the pro- 

 gressive motion by heating to 500° and more. 



In order to put these assumptions to the proof, I have already 

 commenced a series of experiments, in which the temperatures 

 of the discharges in Geissler tubes under various conditions 

 are determined. 



Leipzig, August 1878. 



XII. An easy Method for Adjusting the Collimator of a Spec- 

 troscope. By Arthur Schuster, Ph.D., F.E.A.S.* 



THE ordinary method for adjusting the collimator of a 

 spectroscope for parallel rays is only applicable to the 

 mean rays of an achromatic combination. At the extreme 

 ends of the spectrum a readjustment has to be made. If the 

 ultra-violet rays are observed, and if the lenses are of quartz, 

 the ordinary method cannot be used. The following method 

 is so simple that I cannot help thinking it has often been in 

 use ; yet I have nowhere seen it described, and I know that 

 others, like myself, have often found a difficulty in making 



* Commuuicated by the Physical Society. 

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