Astronomical and Nautical Collections^ I3i5 



to be multiplied by the interval between the two slits, and 

 divided by the distance of the screen from the wires of the 

 micrometer employed for measuring the fringes. 



This formula is also applicable to the dark and bright 

 stripes which are observed in the shadow of a narrow sub- 

 stance, substituting the breadth of this substance for the 

 interval which separates the two slits^ as long as the stripes 

 are far enough from the edges of the shadow : for when 

 they approach very near to the edges, it is shown, both by 

 theory and by experiment, that this calculation does not repre- 

 sent the facts with sufficient accuracy; and it is not perfectly 

 correct in all cases, either for the fringes within the shadow, 

 or. for those of the twjo slits, but only for the fringes pro- 

 duced by the mirrors, which exhibit the simplest case of the 

 interference of rays slightly inclined to each other. In order 

 to obtain from the theory a rigorous determination of the 

 situation of the dark and light stripes in the two former 

 cases, it is not sufficient to calculate the effect of two systems 

 of undulations, but those of an infinite number of similar 

 groups must be combined, according to a principle which 

 will shortly be explained, in treating of the general theory 

 of diffraction. 



ii. Ride for the Correction of a Lunar Obseevation. 

 j?y Mr. William WisEMAti, of Hull, 



Rule. > 



Add together the reserved logarithm (found as directedi 

 page 111 and 1 12 of the Appendix to the third edition of the 

 Requisite Tables) the log. sines of half the sum, and half the 

 difference of the apparent distance, and difference of apparent 

 altitudes, and 0.3010300, the log. of 2. Then, to the natural 

 number corresponding to the sum of these four logarithms, 

 add the natural verse sine of the difference of true altitudes, 

 and the sum will be the natural verse sine of the true distance. 

 . Or, having obtained the natural number, as directed 

 above, subtract it from the natural cosine of the difference of 

 the true altitudes, and the remainder will be the naturs^ 

 cosine of the true distance. 



