VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 353 



voidable defect in it may become invisible, must be determined by experiment. 

 If any variation be made in the form of this lens, it will be better to make the 

 plane side rather a little convex than concave. By the latter the image would be 

 distorted by the too great obliquity of the rays near the extremity of the lens. 

 Thus we have a system of eye-glasses which may be taken out of the tele- 

 scope, in order to wipe them at pleasure. Or the magnifying power of the tele- 

 scope may be varied without affecting the line of collimation, or in any manner 

 altering the adjustment of the instrument to which such telescopes may be applied, 

 with many other advantages. In the present improved state of telescopes too, 

 the disagreeable appearance of the wires from the great power of the eye-glasses 

 is in a great degree remedied. The same principle may be usefully employed in 

 many other cases. What is herein contained is only to be considered as an expla- 

 nation of this very useful construction, and which is given in hopes that some 

 person of more abilities in the science of optics will favour us with a general 

 theorem, in order that its application maybe more universal. 



VI. Account of several Lunar Rainbows. By Marmaduhe Tunstall, Esq. 



F.R. S. p. 100. 



The first iris was seen the 27 th February, 1782, between 7 and 8, in tolerable 

 distinct colours, similar to a solar one, but more faint ; the orange colour seemed 

 to predominate. The night was windy, and though there was then a drizzling 

 rain and dark cloud, in which the rainbow was reflected, it proved afterwards a 

 light frost. 



The 2d lunar iris happened July the 30th, 1782, about 11 o'clock, which 

 lasted about a quarter of an hour, without colours. The last was on Friday, 

 Oct. the 18th, 1782, perhaps the most extraordinary one of the kind ever seen. 

 It was first visible about 9 o'clock, and continued, though with very different 

 degrees of brilliancy, till past 2. At first, though a strongly marked bow, it 

 was without colours; but afterwards they were very conspicuous and vivid in the 

 same form as in the solar, though fainter; the red, green, and purple, were 

 most distinguishable. The wind was very high most part of the time, accompa- 

 nied with a drizzling rain. One particular, rather singular, in the 2d, viz. of 

 July the 30th, was its being 6 days after the full of the moon, and the last, though 

 of so long a duration, was 3 days before the full ; that of the 27 th of February 

 was exactly at the full, which used to be judged the only time they could be seen; 

 though in the Encyclopedic there is an account that Weidler observed one in 17 19, 

 in the first quarter of the moon, with faint colours, and in very calm weather. 



VII. Account of an Earthquake. By John Lloyd, Esq. p. 104. 

 On Saturday Oct. 5, 1782, between 8 and 9 o'clock in the evening, a shock 

 vol. xv. Z z 



