Royal Astronomical Society. 229 



arising from this singularity. It is not worth while to make a for- 

 mal communication of such a thing to the Astronomical Society ; but 

 if you think it worth your verbal mention at the meeting, it may be 

 interesting to those (if any) who are busy about satellitary pertur- 

 bations." 



On a new Double-image Micrometer, communicated in a Letter 

 to the President by Professor Powell. 



" My dear Sir, — The following suggestion for a very simple double- 

 image micrometer occurred to me a few years ago ; but not having 

 much practical acquaintance with these matters, I should hardly have 

 supposed it to possess novelty or prospect of utility enough to render 

 it worthy the notice of the Astronomical Society, had you not en- 

 couraged me to communicate it. 



" The optical principle is merely that of a ray of light, refracted 

 obliquely through a plate of glass with parallel surfaces, and emer- 

 ging parallel to, but not coincident with, its original direction. 



" If, then, such a plate intercept half the cone of rays going from 

 the object-glass of a telescope to its focus, there will be formed, at 

 the focus, besides the direct, a deviated image ; and the angular de- 

 viation will be dependent on the inclination, the thickness, and 

 the refractive power of the glass, involving a constant factor to 

 be found by observation for the particular instrument, agreeably to 

 the following formula, which may be easily tabulated for all incli- 

 nations. 



" If <f> and <p l be the angles of incidence and refraction, and t the 

 thickness of the plate, a moment's consideration will shew that the 

 oblique path of the ray within the plate = t . sec 1 ; and, for the 

 angular space 9 between the direct and the deviated ray, c being the 

 constant for the instrument, we have 



6=c . t . sec0' sin (sin <£— 1 ). 



" If such a plate be placed within the tube of a telescope between 

 the object-glass and its focus, so that a variable inclination can be 

 given to it, and a graduated circle be read off outside ; then, when 

 the plate is perpendicular to the axis there will be no deviation ; but, 

 when it is inclined, the deviation, found as above, will give the 

 measurement of a small angular space, as in other double-image 

 micrometers. 



" The less the thickness of the glass, the greater will be the range 

 of the scale for a very small deviation. 



" The idea has as yet been put to trial only in a very rough man- 

 ner ; and I offer it without at all being able to say whether serious 

 practical difficulties may not arise, which can only be decided on a 

 more accurate construction ; or should no such objection occur, it 

 still remains to be seen whether this suggestion may afford any use- 

 ful addition to the micrometrical resources already in the hands of 

 the observer, so as at least to be available in some cases : but these 

 are points on which the practical astronomer alone can judge ; and it 

 is mainly in the hope that it may receive such examination that I 

 submit this idea to the Astronomical Society. — I remain, &c, 



" Baden Powell, 

 " Oxford, December 7th, 1845." " Savilian Professor of Geometry." 



