118 On the Resolving-power of Telescopes. 



intervals between consecutive lines equal to t»»w and OJrtg . 



1 4575 8425 



According to theory, the minimum angle is approximately 

 equal to that subtended by the wave-length of light, \, at a dis- 

 tance equal to the width of the slit, a. In the present case 

 X=5-89 x 10- 5 centimetres, and a= -107 x 2*54, or -196 x 2-54 

 centimetres, so that 



X_ 1 1 



a ~ 4615 ° r 8413' 



agreeing with the angles found by observation more closely 

 than we should have any right to expect. 



Besides these slits, four circular apertures were examined. 

 Their diameters were measured under a magnifier on a glass 

 scale divided to tenths of a millimetre, and were found to be 

 •172, *315, '48, *63 centimetre respectively. In the case of 

 the two smaller holes the illumination given by an ordinary 

 soda-flame was hardly sufficient ; but with the assistance of a 

 jet of oxygen the observation could be made. 



The following distances are the means of those found by 

 two observers* — 51*5, 98, 149, 196 inches, corresponding to 



angular intervals ^_ „1_ J_ _i_ respectively. If D 



represent the diameter of the aperture, the values of ^r are re- 



1111 D 



spectively ^fi, ^^, g^, JqYoO* The ratios oftlie observed 



angular intervals to ^ are thus 



1-13, 1-09, 1-09, 1*09. 



That a circular aperture would be less effective than a slit 

 of the same width might have been expected. Even in the 

 case of a slit it is advantageous to stop some of the central, in 

 order to increase the relative importance of the extreme, rays; 

 and with a circular aperture the extreme rays are much worse 

 represented than with a slit. From the above results it appears 

 that, to have an equal resolving-power, the circular aperture 

 must be about a tenth part wider than the slit. 



Merely to show the dependence of resolving-power on aper- 

 ture it is not necessary to use a telescope at all. It is suffi- 

 cient to look at wire gauze backed by the sky, or by a flame, 

 through a piece of blackened cardboard pierced by a needle and 

 held close to the eye. By varying the distance the point is 

 easily found at which resolution ceases; and the observation is 

 * Mr. Glazebrook and myself. 



