NATURAL PHILOSOPHY, 169 



dilatation of the cometary volume seems to prevent the comet from 

 sending; out much light, either rejected or direct. 



Applying these principles to nebulae proper, we must conclude that 

 nebulous vapor is necessarily too diffuse, has too little density to In- 

 visible, when far removed from us. According to this, then, nebula' 

 can not in general be visible, unless they are considerably condensed, 

 and perhaps actually converted into stars. It is perfectly evident that 

 the luminosity of nebulous vapor must be very feeble, even where the 

 light is inherent. The process of condensation only, then, can render 

 nebulous matter visible through our telescopes. Will not this account 

 for the fact that higher telescopic power resolves previous nebulae ? 

 It is very doubtful whether our best telescopes will ever be able to 

 bring into view any real nebulae. According to the jS'ebular Hypothe- 

 sis, we should not expect to find any large collection of nebulous 

 matter in the vicinity of our system, either planetary or stellar, and 

 ages may pass before our system, in its progress through space, will 

 come near any of the small patches that may exist, so as to render 

 them visible to us. 



Note. The influence of the magnetic power of the sun, may be a 

 potent cause to render comets visible as they approach the great cen- 

 tral luminary. It is a fact derived from observation, that the sun af- 

 fects the magnetic needle, and that its period is closely connected with 

 the period of the solar spots. It is also known that the auroras influence 

 the needle ; and that they are subject to the same law of periodicity 

 as the solar spots, and thus seem to be connected with solar influence. 

 The. effect of the auroras is evidently light-producing. 



ERRORS IN ASTRONOMICAL OBSERVATIONS OF PHYSIOLOGICAL 



ORIGIN. 



A memoir on this subject, by M. Faye, has been recently read be- 

 fore the Academy of Sciences at Paris. In these observations the 

 human organism no doubt attains by practice to a wonderful degree 

 of precision ; yet the eye, observing by means of a microscope, and 

 the ear by means of a pendulum which vibrates 500 times a second 

 will sometimes fail : and small errors in time lead to great ones in 

 calculation. The human machine is subject to variations due to the 

 disturbances incident to the processes of digestion and circulation, 

 and to nervous fatigue. M. Faye proposes to substitute for the 

 senses, or at least bring to their aid, the two great discoveries of our 

 epoch, photography and the electric telegraph. He was led to take 

 up the subject in consequence of M. M. Plautamour and Hirsch detect- 

 ing errors, which might be ascribed to physiological causes, in their 

 researches to determine the difference of longitude between the ob- 

 servatories of Geneva and Ncufchatel. The possibility of suppressing 

 the observer has been fully demonstrated at Paris, some years ago, 

 in experiments made under M. Faye ? s own direction. The process, 

 which is of extreme simplicity in regard to the sun, becomes more 

 delicate, but not impracticable, when applied to the stars. It consists 

 in substituting for the eye of tlio oVerver a photographic plate, and 

 in registering electrically the moment at which light is admitted into 

 the dark chamber applied to the meridi:v:i glass. M. Faye thus ob- 

 tained ten observations of the sun in twenty seconds. 

 15 



