280 PHILOSOPHICAL TRANSACTIONS. [aNNO I676. 



but if it be a little cloudy, that cannot much prejudice the experiment, so the 

 sun do but shine distinctly through the cloud. 



These things being thus ordered, if the refracted light fall perpendicularly on 

 a wall or paper at 20 feet or more from the prism, it will appear in an oblong 

 form, cross to the axis of the prism, red at one end, and violet at the other; 

 the length five times the breadth, more or less according to the quantity of the 

 refraction; the sides, straight lines, parallel to each other; and the ends con- 

 fused, but yet seeming semicircular. 



I hope, therefore, Mr. Linus's friends will not entertain themselves any further 

 about incongruous surmises, but try the experiment as Mr. Gascoin has pro- 

 mised. And then, since Mr. Gascoin tells you. That " the experiment being 

 of itself extraordinary and surprising, and besides ushering in new principles into 

 optics, quite contrary to the common and received, it will be hard to persuade 

 it as a truth, till it be made so visible to all as it were a shame to deny it :" if he 

 esteem it so extraordinary, he may have the privilege of making it so visible to 

 all, that it will be a shame to deny it. For I dare say, after his testimony no- 

 body else will scruple it. And I make no question but he will hit of it, it being 

 so plain and easy that I am very much at a loss to imagine what way Mr. Linus 

 took to miss. 



Extract of three Letters from Sig. Cassini, on his Observations of the Lunar 

 Eclipse of Dec. 21, l675, and Remarks on Mr. Flamsteed's Account of the 

 same. N° 123, pp. 56l, 563, 564. Translated from the Latin. 



In this eclipse, two of the principal circumstances we have determined exactly, 

 viz. the middle time of the eclipse and its magnitude. The middle is deduced 

 not only from a. comparison of the beginning and end, but also of two equal 

 phases, which are very easily determined, viz. when the distance of the horns 

 was equal to the semidiameter of the moon, taken before the eclipse, being 15' 

 18'^ That is, when the beginning of the eclipse was estimated at 2^ 24™ 35^ 

 after midnight. 



End of the total darkness, a like penumbra being left as was in h. m. s. 



determining the beginning 4 15 25 



Duration of the whole eclipse comes out 1 50 50 



The semiduration therefore O 55 25 



And the middle of the eclipse 3 20 O 



A 6th part of the circumference is cut off 2 38 5 



And, again, end of total obscuration > 4 2 25 



The interval or duration 1 24 20 



