7 6 LIGHT SCIENCE FOR LEISURE HOURS. 



occasionally take for granted and leave unexplained 

 just those matters which the student is best able, as 

 well as most anxious, to comprehend. 



Eclipses certainly seem to me to be a case in point. 

 There is something amusing so at least I conceive 

 in the elaborate care with which the student of the 

 noblest of all sciences is informed that an opaque body 

 can cast a shadow, and that this shadow will have such 

 and such characteristics. I am not here speaking of 

 elementary treatises : it is reasonable enough, perhaps, 

 in a first book for children to explain that ' when the 

 moon stops the sun's light its shadow falls on a part of 

 the earth,' and that 4 the people who live on that par- 

 ticular part of the earth where the shadow falls cannot 

 see the sun because the moon is in the way.' This is 

 very pleasing and instructive for very small people ; 

 but when in treatises of a higher class the student is 

 gravely informed of these things, as though they in- 

 volved entirely new and striking conceptions, the idea 

 is suggested that astronomers think but lightly of the 

 capacity of those who chance not to have made 

 astronomy their chief subject of inquiry. 



On the other hand, the points about which most 

 readers would care to hear something are commonly left 

 untouched. Scarcely any reader of the usual explana- 

 tion of eclipses fails to feel interested in the question 

 of the laws according to which the moon comes between 

 the sun and the earth, or the earth between the sun 

 and the moon. The student feels that it may be very 

 well to show him the consequences which follow when 



