1813.] 



the Rev, Nevil Maskelyne. 



All 



composed of a prism which moves all along the axis of the tele- 

 scope, like those of M. Rochon and P. Boscovich. According 

 to this last philosopher Maskelyne first conceived the idea of 

 such a micrometer. Boscovich affirms that he also conceived it. 

 It is not without example to find the same contrivance fallen 

 upon hy different persons almost at the same time without any 

 communication with each other. Hitherto M. Rochon is the 

 only person who has published observations made with this 

 micrometer. The idea of employing in it double refraction 

 belongs to him incontestably, as Boscovich himself acknow- 

 ledges. Dr. Maskelyne employed only a common glass. It 

 seems certain that he first thought of moving the prism in the 

 interior of the telescope. It remains for us to know the advan- 

 tages which he derived from this construction. 



Dr. Maskelyne, who knew the value of excellent instruments, 

 as he was continually using them, turned his whole attention to 

 preserve them properly, and to improve them by the additions 

 suggested by his experience and skill in optics. He made the 

 eye-piece moveable, in order to avoid all parallax in bringing the 

 eye opposite to each of the five wires which the luminary crosses 

 in succession. He discovered the inconvenience of straight trap- 

 doors used in all observatories. He enlarged the size of those 

 at Greenwich, after having shown the necessity of placing the 

 telescopes as much as possible in the open air. 



Notwithstanding all these cares it has been lately suspected 

 that his quadrant had become less exact in consequence of the 

 friction which it had undergone during its continual employment 

 for more than 50 years. It was very natural that an astronomer 

 who always paid the same degree of attention to his observations, 

 and who did not perceive in his instrument any mark of old age, 

 should not be the first to detect alterations in it very slight in 

 themselves. Other instruments, more modern, and of a diffe- 

 rent construction, and placed in the hands of attentive astrono- 

 mers, occasioned the first suspicions. It is true that the small 

 variations which appear to have been observed may be accounted 

 for in such a way as to acquit the quadrant at Greenwich of 

 inaccuracy. MM. Besset and Oltmanns gave explanations of 

 them not deficient in probability ; but the most certain method 

 was to procure new instruments. This was what Dr. Maskelyne 

 did. He employed the celebrated Troughton to make a grand 

 and superb circle, which he had not the pleasure himself of 

 placing in his observatory; but which he put into the hands of 

 his successor. Mr. Pond will make us acquainted with the 

 faults which age had produced in the Greenwich quadrant, and 

 will inform us what corrections must be made in the latter obser- 

 vations at Greenwich to render them as valuable as the more 

 mx\y observations in the same place. Thus instruments grow 



