SlIai!Mauh1??«'Tu'.] ^oijol Microscopcol Society. 137 



objects not in the centre have a strong bur inwards, the colour 

 is much under-corrected, and the sjjherical 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 pre- 

 sented itself, the combination proved to have the spherical aberra- 

 tion 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 

 To 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 shghtly outwards." 



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 an-angements 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. 

 Eobert Brown, the botanist, he set to work in good earnest to 

 strive to solve the difiicult 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 Chevalier, and of their performance when used singly. 

 The others give the effects of various combinations of those glasses 

 upon the chromatic 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 tables that the per- 

 formance of each combination is given, both when the glasses are 

 close and when they are separated a certain distance from each 

 other. As we look down the tables we seem for a while to find 

 confusion worse confounded. We see, indeed, abundant evidence 

 of the great effect produced both upon coma and upon spherical 



