Yll 



improve the theory. He remarks that there must be a gradual, 

 though rapid, change of phase, instead of the sudden one given by 

 Fresnel's formula, thus seeing faintly a result clearly explained five 

 3'ears after by the theoretical investigations of Green. 



At this period of his life Professor Airy's labours are evidently 

 divided between astronomy and the theory of light. The first was 

 connected with his work at the Observatory, the second with his 

 lectures as Professor. Thus, in 1833, he writes in the 6 Cambridge 

 Transactions' on Newton's experiments in diffraction; in 1835, on 

 the diffraction of an object glass with a circular aperture ; in 1838, on 

 the intensity of light in the neighbourhood of a caustic. In 1840 

 he chose as the subject of the Bakerian Lecture the theoretical 

 explanation of an apparent new polarity in light. 



There is an equally important list of papers on astronomy. In 

 1832 he communicates to the British Association a report on the 

 progress of astronomy during the present century. This was trans- 

 lated into German, three years after, by C. L. Littrow, of the Royal 

 Observatory, Vienna. The Viennese astronomer thinks that Pro- 

 fessor Airy has treated German astronomy like a step-mother, but, 

 nevertheless, he says there is no other work in which the progress of 

 astronomy is so briefly and so accurately given. In 1834 he writes 

 for the 'Nautical Almanac,' on the perturbations of small planets 

 and comets of short period. There is more than one paper on the mass 

 of Jupiter. In 1834 he writes a paper, for the Astronomical Society, 

 on the solar eclipse of July 16, 1833, which was seen extremely well 

 at Cambridge. On this occasion he adopted a new plan of observa- 

 tion ; instead of noting the times of the beginning or the end, he so 

 chose the quantities to be measured that any errors in the elements 

 would be observed after they had been largely multiplied. For 

 example, at the beginning of the eclipse, when the discs of the Sun 

 and Moon only slightly overlap, it is obvious that the length of the 

 straight line joining the cusps is much more affected than the versine 

 by any small error in the angular distance of the centres of the discs. 

 To detect such errors, the attention of the observer should be directed 

 to the length of this line. In like manner, the whole duration of the 

 eclipse was divided into periods, for each of which he arranged' 

 appropriate measures. 



These papers, too numerous to catalogue in this place, did not 

 exhaust the energy of the Professor, for he found time to publish 

 treatises on Trigonometry, the Figure of the Earth, and one on 

 Gravitation. The latter was written for the ' Penny Cyclopedia,' 

 but previously published, in 1834, for the use of students in the 

 University of Cambridge. It was an attempt to explain the per- 

 turbations of the solar system without introducing an algebraic 

 symbol. Having thus denied himself the use of the most powerful 



