20 Mr. A. A. Michelson on Astronomical Measurements. 



are absent in the case of the planet, it may be considered 

 certain that the cause of the disappearance in the latter case 

 is its appreciable disk. 



The angular diameter of the latter can be found (on the 

 supposition of uniform illumination) by the formula 



-I* \ 



where \ is the wave-length of the light, and b the distance 

 between the centres of the mirrors a and b *. 



From what has gone before,, it will be inferred that the 

 chief object of the method proposed is the measurement of 

 the apparent size of minute telescopic objects, such as plane- 

 toids, satellites, and possibly star disks, and also double stars 

 too close to be resolved in the most powerful telescope. But 

 it is clear that interference methods may also be employed 

 for the measurement of star-places. 



Thus, in observations for right ascension, the slits would be 

 placed parallel to the meridian, and the instant of passage of 

 the central white fringe across the spider-lines noted ; and 

 in observations for declination, the slits would be horizontal, 

 the cross-hair being brought to coincide with the centre of 

 the white fringe. 



The increase in accuracy to be expected from this method 

 would, however, be limited by the imperfections of our 

 present means of measuring time and angles ; still, it would 

 appear that by its use a one-inch glass may be made to do 

 the work now required of a ten-inch. 



Conclusion. 



(1) Interference phenomena produced under appropriate 

 conditions from light emanating from a source of finite mag- 

 nitude become indistinct as the size increases; finally vanishing 

 when the angle subtended by the source is equal to the 

 smallest angle which an equivalent telescope can resolve, 

 multiplied by a constant factor depending on the shape and 

 distribution of light in the source and on the order of the 

 disappearance. 



(2) The vanishing of the fringes can ordinarily be deter- 

 mined with such accuracy that single readings give results 

 from fifty to one hundred times as accurate as can be obtained 

 with a telescope of equal aperture. 



* Better, the distance between the centres of the apertures in front of 

 these mirrors. 



