172 COSMOS. 



another on the crystalline lens, they necessarily move 

 according to the direction in which the head is in- 



evidemment de viser a des etoiles excessivement rapprochees 

 entr'elles et de voir si dans les etoiles doubles connues leg 

 images se confondent, si elles empietent Tune sur 1'autre, ou 

 bien si on les apergoit bien nettement separees." 



' The principal causes of indistinct vision are : aberration of 

 the sphericity of the eye, diffraction at the margins of the 

 pupil,and irritation transmitted to contiguous points of the retina. 

 Indistinct vision exists where the focus does not fall exactly on 

 the retina, but either somewhat before or behind it. The tails 

 of the stars are the result of indistinctness of vision, as far as it 

 depends on the constitution of the crystalline lens. According 

 to a very old paper of Hassenfratz (1809) ' the 4 or 8 tails 

 which surround the stars or a candle seen at a distance 

 of 25 metres [82 feet], are the caustics formed on the crystal- 

 line lens by the intersection of refracted rays.' These caustics 

 follow the movements of the head. The property of the tele- 

 scope in giving a definite outline to images, causes it to con- 

 centrate in a small space, the light which would otherwise be 

 more widely diffused. This obtains for the fixed stars and for 

 the discs of planets. The light of stars having no actual 

 discs, maintains the same intensity, whatever may be the mag- 

 nifying power of the instrument. The aerial field from which 

 the star is projected in the telescope is rendered more black 

 by the magnifying property of the instrument, by which the 

 molecules of air inc'uded in the field of view are expanded. 

 Planets having actual discs become fainter from this effect of 

 expansion. When the focal image is clearly defined, and 

 when the rays emanating from one point of the object are con- 

 centrated into one point in the image, the ocular focus affords 

 satisfactory results. But if, on the contrary, the rays ema- 

 nating from one point do not reunite in the focus into one 

 point, but form a small circle, the images of two contiguous 

 points of the object will necessarily impinge upon each other ; 

 and their rays will be confused. This confusion cannot be 

 removed by the ocular ; since the only part it performs is that 

 of magnifying. It magnifies everything comprised in the 

 image, including its defects. As the stars have no sensible 

 angular diameters, those which they present are principally 



