ioo PLEASANT WA YS IN SCIENCE. 



known facts. Of course these results are open to the ob- 

 jection that the observers have known beforehand what to 

 expect, and that expectation often deceives the mind, 

 especially in cases where the thing to be observed is not 

 at all easy to recognize. It will presently be seen that the 

 new method has been more satisfactorily tested, in this 

 respect, in other ways. It may be partly due to the effect 

 of expectation that in the case of Venus the motions of 

 approach and recession, tested by the new method, have 

 always been somewhat too great A part of the excess 

 may be due to the use of the measure of the sun's distance, 

 and therefore the measures of the dimensions of the solar 

 system, in vogue before the recent transit These measures 

 fall short to some degree of those which result from the 

 observations made in December, 1874, on Venus in transit, 

 the sun's distance being estimated at about 91,400,000 miles 

 instead of 92,000,000 miles, which would seem to be nearer 

 me real distance. Of course all the motions within the 

 solar system would be correspondingly under-estimated. On 

 the other hand, the new method would give all velocities 

 with absolute correctness if instrumental difficulties could 

 be overcome. The difference between the real velocities 

 of Venus approaching and receding, and those calculated 

 according to the present inexact estimate of the sun's dis- 

 tance, is however much less than the observed discrepancy, 

 doubtless due to the difficulties involved in the application 

 of this most difficult method. I note the point, chiefly for 

 the sake of mentioning the circumstance that theoretically 

 the method affords a new means of measuring the dimen- 

 sions of the solar system. Whensoever the practical appli- 

 cation of the method has been so far improved that the 

 rate of approach or recession of Venus, or Mercury, or 

 Jupiter, or Saturn (any one of these planets), can be 

 determined on any occasion, with great nicety, we can at 

 once infer the sun's distance with corresponding exactness. 

 Considering that the method has only been invented 

 ten years (setting aside Doppler's first vague ideas respect- 

 ing it), and that spectroscopic analysis as a method of exact 



