OF THE COSMOS. PAEALLAX OF STARS. 185 



rays of light sent forth (luminous molecules) are so held 

 back by the force of attraction, that they cannot pass 

 beyond a certain limit. ( 304 ) If, as may well be assumed, 

 there are dark invisible bodies in space in which the process 

 of light-producing undulations does not take place, they 

 must either not fall within the circumference of our planetary 

 and cometary system, or they must be of very small mass, 

 since their presence does not manifest itself by any percep- 

 tible perturbations. 



The investigation of the motion of the fixed stars in 

 amount and direction (meaning thereby both their true 

 proper motion, and the merely apparent motion due to the 

 change in the observer's place caused by the earth's motion 

 in her orbit), the determination of the distance of the fixed 

 stars from the Sun by investigations of their parallax, and 

 conjectures as to the part of space towards which our plane- 

 tary system is moving ; are three problems in Astronomy 

 which, in respect to the means of observation which have 

 been successfully employed in their partial solution, are 

 nearly allied to each other. Every improvement, either of 

 instruments or of methods, applied to the advancement of 

 one of these difficult and complicated inquiries, has been 

 productive of benefit to the others. I prefer commencing 

 with the parallaxes, and with the determination of the 

 distances of some of the fixed stars, in order to complete the 

 account of the present state of our knowledge in reference 

 to single fixed stars. 



As early as the beginning of the 17th century, Galileo put 

 forward the idea of " measuring the, doubtless very unequal, 

 distances of the fixed stars from the solar system" ; and even 



