100 UPON INSTRUMENTS CALLED PERISCOPIC. 



the numerical coefficient found by Burg gives -y4 T for the 

 oblatenessj but if this coefficient be diminished by 0,665, 

 then the oblateness becomes • s 4~j so that a variation even to 

 this small amount in the coefficient augments the denominator 

 of the oblateness nearly ^ part. 



The same happens with regard to the pendulum vibrating 

 seconds ; for, supposing its length at 45° to have been cor 

 rectly ascertained by MM. Biot and Mathieu, if we wish 

 to know the length of a second's pendulum at the equator, 

 corresponding to an oblateness of lr fa t we find it to be 439, 1810 

 . lines. Now this length differs from that determined by Bou- 

 guer only by 0,029 °f a * me J an( ^ M. Laplace even think* 

 that the result of Bouguer should be diminished by about 

 double this quantity. We see from hence how much thesd 

 little differences, whether produced by errors of observation, 

 or irregularities in the earth itself, are liable to affect the deno- 

 minator of the fraction expressing the oblateness. 



Fortunately, it seems probable, that the utmost latitude of 

 our present uncertainty is between the limits of 330 and 310, 

 and the mean of these may be considered as a very near ap- 

 proximation to the truth. 



in. 



Critical Observations, on Dr. JVollaston's stated improvement of 

 the Camera Obscura and Microscope in the application of the 

 Meniscus, and Two Piano-Convex Lenses ; proving their in- 

 feriority to the double Convex Lens generally used. By Mr. 

 William Jones, Optician. 



To Mr. Nicholson. 

 SIR, 



Reference to a A S my observations, published in your Journal, Volume 7, 



former of the jM\_ pr0V ed, I trust, that the Periscopic Spectacle Glass, ad- 

 author upon ■ l ' ^ ; . r . . . . 



periscopic vertised by Dr. Wollaston, as possessing a new optical pnnci- 

 epecucle». pj^ 



