MEMOIR ON ACHROMATIC GLASSES. U9 



Ships, buildings in which certain rules little liable to varia- 

 tion areobferved, and the dimenfions of different animals, may 

 ferve to give this diftance. By means of fuch objecls I have 

 often determined inacceffible diftances with truly furprifing 

 celerity, by means of a portable telefcope with prifms, which 

 requires no fupport for taking angles. When greater accu- 

 racy is required, objects of which the diameter is well afcer- 

 tained mull be employed. 



If lighthoufes had on them a crofs of fixed and known di- Croffes of known 

 mentions, it would thus be of great fervice to navigators : j* 1 ™ 6 * 1 *" 101 " on 



r r i- *i • • 11. hght-houfes 



for by the angle of the perpendicular piece, given by this would be of 

 micrometer, the diftance of the (hip from the lighthoufe would & tz } u f iut y t0 

 be afcertained ; and by that of the horizontal piece would be 

 known the poiition of the fhip with refpecl to the coaft. 



QUESTION II. 



The dijlance of an object being given, to determine its magnitude. 



This is precifely the reverfe of the preceding. The magnitude 



of an objeft de- 



QUESTION III. SSSS2T 



The magnitude and diftance of an objeel being imknoivn, to deter- known. 



mine both. 



Suppofe the objeel makes an angle of 40' with my eye ; as Method of de- 



this angle will increafe in regular proportion as the diftance is SS^3j 



diminifhed, I approach the objeel till the angle is 41', a d if- diftance, where 



ference which will be fufficient in ordinary cafes. If I find , both are un " 



/- known, 



the diftance between the two (rations, on meafuring it, to be 



a hundred toifes, I (hall obtain the whole diftance by multi- 

 plying this by the number of minutes of the larger angle, and 

 dividing the producl by the difference between the two an- 

 gles, which will give in this inftance 4100 toifes. The dif- 

 tance being thus obtained, the magnitude of the objeel will 

 be found by dividing 4100 toifes by 83f , the number anfwer- 

 ing to 41'; which will give the magnitude forty-nine toifes 

 feven inches nearly. 



XI. Objervations 



