300 THE PRINCIPLES OF SCIENCE. 



by Lord Rosse's telescope possesses some analogy to what 

 would happen in a system revolving in a dense retard- 

 ing medium. Let us once ascertain by the spectroscope 

 that there is a dense envelope of gas, and the forms of 

 those bodies are at once brought into harmony with the 

 laws of matter on this globe. Viewing such worlds as 

 we do from a fixed distant point, they appear variously 

 distorted according to the laws of perspective ; but when 

 we find in many objects forms which might have pro- 

 ceeded from the same object variously inclined to the 

 line of vision, analogy will aid us in determining the 

 real form. Thus when we see an apparent nebulous 

 ring, we may be unable to decide whether it is really 

 a ring of matter or a spherical shell, of which the ob- 

 liquely seen edges are alone apparent. But if elsewhere 

 we discover, as did Lord Rosse, another nebula present- 

 ing the distinct appearance of a ring seen edgeways, we 

 may infer with some probability from one case to the 

 other. By similar processes of comparison and analogical 

 reasoning, we may in time assign with much confidence 

 the absolute forms of many classes of celestial objects 111 . 



In speculations concerning the physical condition of 

 other planets and heavenly bodies, we must often depend 

 upon analogies of a very weak character. We may be 

 said to know that the moon has mountains and valleys, 

 plains and ridges, volcanoes, and streams of lava, and, 

 in spite of the absence of air and water, the rocky sur- 

 face of the moon presents so many familiar appearances 

 that we do not hesitate to compare them with the features 

 of our own globe. We infer with high probability that 

 Mars has polar snow and an atmosphere absorbing blue 

 rays like our own ; Jupiter undoubtedly possesses a cloudy 

 atmosphere, possibly not unlike a magnified copy of that 

 surrounding the earth, but our tendency to adopt an- 

 m Grant's 'History of Physical Astronomy,' pp. 570-571. 



