546 OBITUARY NOTICE OF JOSEPH JACKSON LISTER 



the advantages resulting from this combination,' he ' tried some others ', among 

 the rest two of Tulley's triple glasses, each of which taken singly was of fine 

 performance. But, instead of unmixed improvement resulting, we find it noted : 

 ' N.B. — Each glass separately shows a bright object all over the field without 

 bur, and is nol far from being achromatic. But, combined, the objects not in 

 the centre have a strong bur inwards, the colour is much under-corrected, and 

 the spherical aberration is not right.' 



In the following year we find similar anomalous appearances recorded. 

 Thus, on one occasion, on using in combination a triple glass of Tulley's free 

 from coma and otherwise excellent, and a double plano-convex in which, when 

 used alone, the spherical aberration was rather under-corrected and an outward 

 coma presented itself, the combination proved to have the spherical aberration 

 rather over-corrected and showed an inward coma. Again, a bi-convex glass 

 of Herschel's construction, consisting of a bi-convex of plate with a flint meniscus, 

 when used alone with the flint surface foremost had little or no coma, but when 

 combined with a triple {^ free from coma, showed a ' bur much inwards'. The 

 same glass used alone with the plate side foremost showed a ' bur inwards ', 

 but when it was combined with the triple, which had before had the effect of 

 inducing an inward coma, the bur inwards was changed to a ' bur slightly out- 

 wards '. 



Such are samples of the perplexing and seemingly inconsistent observations 

 recorded at this period. To a less accurate observer and a less acute mind 

 they must have proved utterly bewildering. But he did not despair of finding 

 an explanation of the appearances, and the last note on the subject in that year 

 alludes to the angle formed by the rays of light with the concave lens as affecting 

 the direction of the coma. 



He was afterwards occupied for a while with planning triple glasses to be 

 used in front of the previous triples of Tulley, and with general arrangements 

 for the instrument. But, in November 1829, a set of five plano-convex glasses 

 manufactured by Utzschneider and Fraiinhofer, very similar to those of Chevaher 

 but uncemented, having been placed freely at his disposal by Mr. Robert Brown, 

 the botanist, he set to work in good earnest to strive to solve the difficult problem. 

 The experiments made with this object are recorded in a series of tables, the 

 first of which gives an accurate description of each of the five new glasses and 

 also of those of Chevaher, and of their performance when used singly. The 

 others give the effects of various combinations of those glasses upon the chro- 

 matic and spherical aberrations and upon coma. He had previously observed, 

 as mentioned in a note in 1827, that in a particular combination of two glasses 

 the coma was diminished by separating the glasses. And we find in these 



