VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 203 



purpose of these measures. The appearance of a Lyrae in my Newtonian re- 

 flector with a power of 460 is represented in fig. 2; with 2011) in fig. 3; with 

 3l68 in fig. 4; and with 6450 in fig. 5. Now in all these figures we see, that 

 the centres are still distinct enough to measure their distances with sufficient 

 truth; or if any little error should be introduced by the magnitude of the central 

 point, it will be more than sufficiently balanced by the largeness of the scale. 

 In this manner, with a power of 3l68, I have obtained a scale of no less than 

 10 inches -fig- for the distance of the centres of the two stars of « Geminorum; 

 and as we know these centres to be but a few seconds distant, it is plain how 

 great an advantage we gain by such an enlarged scale. 



These experiments have but very lately pointed out to me a method of making 

 a new micrometer, on a construction entirely different from any that are now in 

 use, which I have been successful enough to put in practice, and by which I 

 have already begun to determine the distance of the centres of some of the most 

 remarkable double stars to a very great degree of accuracy.* 



The powers that may be used on various double stars are different, according 

 to their relative magnitudes: e Bootis, for instance, will not bear the same power 

 as a, Geminorum, nor would it be difficult to assign a reason for it; but as I 

 here shall merely confine myself to facts, it will be sufficient in general to men- 

 tion, that two stars, which are equal, or nearly so, will bear a very high power: 

 with a Geminorum I have gone as far as 31 68; but with the former only to 

 2010. The difficulty of using high powers is exceedingly great; for the field of 

 view takes in less than the diameter of the hair or wire in the finder, and the effect 

 of the earth's diurnal motion is so great, that it requires a great deal of practice 

 to find the object, and manage the instrument. It appears to me very probable, 

 that the diurnal motion of the earth will be the greatest obstacle to our progress 

 in magnifying, unless we can introduce a proper mechanism to carry our tele- 

 scopes in a contrary motion. 



Though opticians have proved that 2 eye-glasses will give a more correct image 

 than one, I have always, from experience, persisted in refusing the assistance of 

 a 2d glass, which is sure to introduce errors greater than those we would correct. 

 Let us resign the double eye-glass to those who view objects merely for entertain- 

 ment, and must have an exorbitant field of view. To a philosopher this is an 

 unpardonable indulgence. I have tried both the single and double eye-glass of 

 equal powers, and always found that the single eye-glass had much the supe- 

 riority in point of light and distinctness. With the double eye-glass I could not 

 see the belts on Saturn, which I very plainly saw with the single one, I would 

 however except all those cases where a large field is absolutely necessary, and 

 where power joined to distinctness is not the sole object of our view. 



* For a description of this micrometer see a subsequent paper. 



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