VOL. I.] PHILOSOPHICAL TRANSACTIONS. 43 



was said, incomparably greater. But yet we see not any faint light beyond the 

 section of the light, which is almost every where equally strong, and we there 

 distinguish nothing at all, not so much as that clearest part called aristarchus, 

 or porphyrites, as I have often tried ; although one may there see the light 

 which the earth sends thither, which is sometimes so strong, that in the moon*s 

 decrease I have often distinctly seen all the parts of the moon that were not 

 enlightened by the sun, together with the difference of the clear parts and the 

 spots, so far as to be able to discern them all. The shadows also of all the 

 cavities of the moon seem to be stronger than they would be if there were a 

 second light. For although afar off the shadows of our bodies, environed with 

 light, seem to us almost dark ; yet they do not appear so in the same degree as the 

 shadows of the moon ; and those on the edge of the section should not ap- 

 pear in the like manner. But I will determine nothing of any of these things. 



To measure Distances at one Station. By M. Auzout. N" 7, p, 124. 



It is long since I found out a method of measuring, with a large telescope, 

 from one station, the distance of objects on the earth. The practice indeed 

 <loes not altogether answer the theory, because the length of the telescopes 

 admits of some latitude ; yet it comes near enough, and is perhaps as just as 

 most of the ways commonly used with instruments. If we consider the sole 

 theory only, an ordinary telescope may be used, having its eye-glass convex : 

 for, by putting the glasses at a little greater distance than they are, propor- 

 tionably to the distance for which it is to serve, and by adding to it a new eye- 

 glass, the object will be seen distinct, though obscure ; and if the eye-glass be 

 convex, the object will appear erect. It may be done two ways ; either by 

 leaving the telescope in its ordinary situation, the object-glass before the eye- 

 glass ; or by inverting it, and putting this before that. But if two object- 

 glasses be used, of which the foci are known, the distance of them will be also 

 known. If we suppose the focus of the first to be B, that of the second C, 

 and the given distance B-J-2D, and that D minus C is equal to F, for this 

 distance will be equal to B-f-C+F— r F^— C^ And if you have the focus of the 

 first object-glass equal to B, the distance at which the second glass is to be put, 



equal to B+C-f D, the focus of the second glass will be found equal to 

 CD . . 



nxr\ ' -^^ if yo^ would have the object magnified as much with these two 



glasses, as it would be with a single one, of which the focus should be of the 

 given distance, having the focus of the object-glass given equal to B, and the 

 distance given equal to B+D ; then the distance between the first and second 



F 2 



