248 



Monday, Feb. 25, 1833. — The evening cleared off, and was very 

 fine from 8 till nearly 11 o'clock p.m. At 7, I placed the instrument 

 on its stand at 9, the thermometer was 37° 6, the barometer 29*32, 

 and the hygrometer "771 ; and the wind was at S.E. 



1 . The Moon. — The examination of the lunar cavities and shadows 

 was rather unsatisfactory. Under the powers 250 and 400, it bore the 

 whole aperture ; but with 90 and 150, there were two spectra, one of 

 which haunted the centre. In definition, the fluid was excelled by the 

 flint-glass, both instruments being very steady. 



2. The great Nebula in Orion. — This mass was seen very fairly with 

 the whole aperture ; and the trapezium was beautifully distinct under 

 all the powers except that of 400. P>om the examination of this ob- 

 ject, the best performance seems to be with the eyepieces 150 and 

 250. The relative light of the flint-glass and the fluid-refractors, when 

 the latter was reduced to six inches of aperture, appeared very nearly 

 equalized. 



3. Venus. — This trial was altogether unsatisfactory, from the strong 

 irradiation and the quantity of loose light. The planet was, however, 

 low down in the west, in a stratum of mist. The only power used 

 was the one of 90 times ; but there was a great defect in distinctness. 



4. Rigel. — This star was in the S.W., and rather low ; it was there- 

 fore, as might have been expected, surrounded with teasing rays, 

 through which I had some difficulty in detecting the small companion. 

 The star had a spurious but broken disc, and was full of colours in 

 every part of the field except the centre, where they were partially 

 destroyed. Powers 150 and 250. 



Tuesday, Feb. 26, 1833. — At 9 in the morning, with the thermo- 

 meter at 38 c, 8, and the hygrometer *798, I examined an enamelled 

 watch-face, which is firmly fixed upon a distant chimney of solid con- 

 struction; and though the solar focus could not be used, I considered 

 sufficiently distinct vision would be obtained to test the achromaticity 

 of the telescope. The plate itself bore the trial better than did the 

 edges of the chimney-sides, where the focus could not be adjusted so 

 as to prevent the alternate production of light green and purple mist, 

 as the eye-tube was pushed in or out : and these colours scarcely dis- 

 appeared, even when brought into the centre of the field of view. Some 

 of this might probably be corrected by adjusting the fluid-lens for near 

 objects : and Professor Barlow writes to me, that this can readily be 

 done ; but that he took off the screw- head, by which it is effected, to 

 prevent mere lookers on from deranging the instrument. The watch- 

 face being upon a dark ground, I played the eye-tube till I procured 

 a spurious disc over it, by which I was satisfied that the centering was 

 very nearly accurate. 



I then left the telescope in statu quo, and at half-past 12 again in- 

 spected it, the thermometer being 47°'6, the barometer 29'23, the hy- 

 grometer -789, and the wind S.S.W. ; particulars which I carefully 

 noted, for a reason which will presently appear. The watch-plate was 

 now considerably plainer, and its figures more sharp and distinct; but 

 the focus required shortening in ; and though there was less colour than 

 before, I was surprised to find it verging to the prismatic extreme, and 

 tinged with red ; a circumstance which ocular foci, or the distance, 



