39 



converging lenses, and augmented by diverging lenses. This 

 action of lenses will depend on their focal-length and not on 

 their diameters or curvature ; the shorter the focal-length, the 

 more decisive it is. The theoretical difncultv, and practical 

 remedy lie in the action of lenses, as affecting the action of 

 vision though the telescope. The error produced in astrono- 

 mical observations by what is called irradiation, springs from 

 two causes, essentially distinct, viz., the ordinary irradiation 

 now described, and the aberration of the instrument. The 

 part of the total error due to irradiation properly so-called, 

 depends on the magnifying power of the eye-piece, the bright- 

 ness of the image, and state of the eye of the observer, and 

 is greatly diminished by the action of the eye-piece in propor- 

 tion to its magnifying power, and its convergency as a lens, 

 varying, however, with the state of the observer's eye. This 

 portion of the total error is made to disappear, when a double 

 image micrometer is made use of, and the observation is only 

 slightly affected with the Heliometer. The Astronomer Royal 

 recommends that the cusps of Venus should be measured as 

 rapidly as possible, and much information as to the distance 

 of the planet's centre from the sun's limb, will be gained in 

 this way. The remaining portion of the total error, viz., that 

 which originates in the aberration of the telescope, will vary 

 with the quality of the instrument, but it will be constant 

 for the same telescope ; it will therefore appear possible, even 

 in the case of an imperfect instrument, with an eye sensi- 

 tive to irradiation, to obtain means of freeing observation from 

 the effect of this peculiar error. In many cases observations 

 may in themselves possess the means of eliminating any error 

 that might arise from personality, by not making contacts 

 too deep or too shallow ; or from the effects of greater or less 

 irradiation due to the instrument or to the eye. Personal 

 errors may also be eliminated by observing both limbs of the 

 planet in the series. 



There are other apparent difficulties attending the observa- 

 tion of the transit, independent of irradiation. Sir Isaac Newton 

 demonstrated that,as a consequence,the poles of the earth must 

 be flattened. About two hundred years ago, Picard found that 

 the pendulum of his transit clock, which beat seconds at the 

 Paris Observatory, must be shortened to beat seconds at 

 Cayenne, near the equator. Subsequent experiments have 

 shown the same results, confirming the fact that the earth 

 is flattened at the poles, giving an elliptic shape to any 

 meridian coinciding with the sea level. A short time ago the 

 problem was again renewed, proving the equatorial curve 

 itself at the sea level to be an ellipse, having a major axis 

 8,800 feet, or 1 — 2S miles, longer than the minor axis. The 



