22 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1665. 



for six months together, the course of irregular tides about the quarter moons, is, 

 to run all day, that is, twelve hours, as from about Q-l to 94^, or 10^ to 10-l, &c. 

 eastward, and all night, that is, twelve hours more, westward ; but during the other 

 six months, from the autumnal to the vernal equinox, the current runs all day 

 westward, and all night eastward. 



This, though I had not the opportunity to be an eye-witness as of the other, 

 I do not at all doubt, having received so credible information of it. 



M. Auzout's Opinion respecting the Apertures of Object Glasses, and 

 their relative Proportions, tvith the several Lengths of Telescopes. 

 N"" 4, p. 55. 



This author observing in a small French tract, lately written by him to one 

 of his countrymen, that large optic glasses have hardly ever so great an aper- 

 ture as small ones, in proportion to what they magnify, and that consequently 

 they must be more dim ; takes occasion to inform the reader, that he has dis- 

 covered that the apertures, which optic glasses can bear with distinctness, are 

 in about the subduplicate proportion of their lengths ; of which he tells us he 

 intends to give the reason and demonstration in his Dioptrics, which he is now 

 writing. In the mean time, he presents the reader with a table of such aper- 

 tures, as follows : 



A Table of the Apertm'es of Object-Glasses. 



