558 PHILOSOPHICAL TRANSACTIONS. [aNNO 1779. 



able parallel wires, he cannot measure any diameter of a planet, except that 

 which is at right angles to the direction of its apparent motion, he cannot with- 

 hold his preference to that construction which measures the angle by the separa- 

 tion of the images. It appeared therefore a matter of some importance, to in- 

 vestigate the causes of the uncertainty which has been found in the observations 

 made with the micrometer with a divided object-glass. The result of my exami- 

 nation convinced me, that were it possible to execute the construction of that 

 micrometer with the degree of accuracy required, it must still be subject to inac- 

 curacy from its principle. 



By the position of the micrometer, every error of its glass is magnified by the 

 telescope; and if each surface of the micrometer glass has not, in every part, 

 precisely the same radius, which opticians must allow to be exceedingly difficult 

 to give, there will be a considerable error in the angle to be measured; and the 

 eye applied to the different parts of the pencil will, without moving the micro- 

 meter, see the images of the object in the telescope fluctuating, sometimes ap- 

 pearing to overlap, and sometimes to separate from each other. But supposing 

 the glass itself to be perfect in its substance and in its curvature, there will still 

 remain imperfections which arise from its principle. A micrometer glass applied 

 to a telescope causes a very considerable aberration. If the focus of the glass is 

 positive, the extreme aberration will be within the geometrical focus; if negative, 

 it will be beyond it; and the aberration not only affects the distinctness of the 

 image, but also the angle measured by the micrometer. 



At the time I took up this subject, the divided object-glass micrometer was 

 the only one which measured angles by the separation of 2 images. Since that 

 time, a very ingenious application of the prism to this purpose has been invented 

 by the Rev. Dr. Maskelyne, Astronomer Royal ; and though experience has not 

 yet ascertained the extent of its merit, it will always deserve great consideration 

 from its ingenuity ; but the morel considered the subject, I became more fully 

 convinced, that the principle of reflection applied to micrometers would have 

 great advantages over those hitherto constructed on the principle of refraction; 

 and the catoptric micrometer I have the honour to describe, besides the advan- 

 tage it derives from the principle of reflection, of not being disturbed by the 

 heterogeniety of light, avoids every defect of other micrometers, and can have 

 no aberration, nor any defect which arises from the imperfection of materials, 

 or of execution, as the extreme simplicity of its construction requires no addi- 

 tional mirrors or glasses to those required for the telescope: and the separation 

 of the image being effected by the inclination of the 1 specula, and not depend- 

 ing on the focus of any lens or mirror, any alteration in the eye of an observer 

 cannot affect the angle measured. It has, peculiar to itself, the advantages of 

 an adjustment to make the images coincide in a direction perpendicular to that 

 of their motion; and also of measuring the diameter of a planet on both sides 



