lSo Trench National Inflittite. 



double refraclhn for the accurate meafurement offmaU angles: 

 by C. WJcbon, — After an interefting hiftorical notice, the 

 abject of vvhich is, ift, To claim the invention of achromatic 

 glafiTes tor the learned Morehall, who, about the year 1743, 

 found means to deftroy the aberration of fphericity and re- 

 frangibility bv employtijg fubftances of different refracting 

 powers : 2d, To call to remembrance what we are indebted 

 to the learned refearehes of drier on thisfubject, C. Rochon 

 announces that he lias improved the inftrument defcribed in 

 his Refearehes on Mechanics and Phyfics, printed in 1783. 



Beccaria has ihown that rock cryftal may be cut into- 

 prifms in fuch a manner as not to produce double refraction. 

 C. Rochon has taken advantage of this difcovery to fubftitutc 

 a prifm of that kind in the room of two variable prifms of 

 glafs wUk-h he employed in his fir ft con ft ruction. 



A new improvement enabled him to obtain the meafurc 

 of angles greater than the diameter of the fun by means of 

 two prifms of rock cryital producing double refraction; when, 

 on again examining Kuler's theory on the aberration of glafTes, 

 he found that it was polrrble by thefe means to meafure by 

 his inftrument angles of a more confiderabre extent; fo that 

 at prefent it can be conveniently employed for determining 

 the respective diftanecs of mips, and the greater or lefs varia- 

 tions of thefe diftances: an advantage obtained but imper- 

 fectly bv Ramfden's inftrnment, which is much ufed in the 

 Englifh and Spanifh navies. 



Decifive experiments have proved to C. Rochon the ad- 

 vantages of his inftrnment over thofe of the celebrated Englifh 

 aTtift. He admits the great difficulty that may fometimes 

 occur in conftructing the inftrument, to thofe who are not 

 well acquainted with the method of cutting rock cryftal. He, 

 however, hopes that this difficulty will be furmounted ; and 

 C. Narci, attached to the agency of the mines, has already 

 conftruclcd the inftrument with all that degree of precifion 

 which can he defired. 



On the co-re Uit ion of geometrical figures : By Z/. JV. ilf, 

 Carnot. — The object of this work is to give a more clear 

 theory of pofitive, negative, and imaginary quantities; and 

 to furnifh more general and certain rules for the changes 

 which a formula eftablifhcd on a primitive fy ft em of points 

 and lines may experience, and of which fome parts change 

 their refpective pofitions. Means are propofed, alio, for 

 giving the enunciation of geometrical problems a technicaf 

 rorm'in order to fhorten them, and render the application of 

 them cafier. 



To illuftrate his method,, and (how the ufe that maybe 



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