76 PHILOSOPHICAL TRANSACTIONS. [aNNO IJQl. 



or hairs, variously disposed, and situated within the telescope, just where the 

 image of the object is formed. To determine an angle with those micrometers, 

 a good deal of calculation is generally required. The micrometers of the other 

 sort, of which there is a great variety ; some being made with moveable parallel 

 wires, others with prisms, others again with a combination of lenses, and so on; 

 are more or less subject to several inconveniences, the principal of which are the 

 following. 1st. Their motions commonly depend on the action of a screw, and 

 of course the imperfections of its threads, and the greater or less quantity of 

 lost motion, which is observable in moving a screw, especially when small, oc- 

 casion a considerable error in the mensuration of angles. 2dly, Their compli- 

 cation and bulk renders them difficultly applicable to a variety of telescopes, 

 especially to the pocket ones. 3dly, They do not measure the angle without 

 some loss of time, which is necessary to turn the screw, or to move some other 

 mechanism. 4thly, and lastly. They are considerably expensive, so that some 

 of them cost even more than a tolerably good telescope. 



After having had long in view the construction of a micrometer, which might 

 be in part at least, if not entirely, free from all those objections ; and, after 

 various attempts, Mr. C. at last succeeded with a simple contrivance, which, 

 after repeated trials, has been found to answer the desired end. This micro- 

 meter, in short, consists of a thin and narrow slip of mother of pearl finely 

 divided, and situated in the focus of the eye-glass of a telescope, just where the 

 image of the object is formed. It is immaterial whether the telescope be a re- 

 fractor or a reflector, provided the eye-glass be a convex lens, and not a concave 

 one, as in the Galilean construction. 



The simplest way of fixing it, is to stick it on the diaphragm, which generally 

 stands within the tube, and in the focus of the eye-glass. When thus fixed, if 

 we look through the eye-glass, the divisions of the micrometrical scale will ap- 

 pear very distinct, unless the diaphragm is not exactly in the focus ; in which 

 case the micrometrical scale must be placed exactly in the focus of the eye-glass, 

 either by pushing the diaphragm backwards or forwards, when that is practicable; 

 or else the scale may be easily removed from one or the other surface of the dia- 

 phragm by the interposition of a circular piece of paper or card, or by a bit of 

 wax. This construction is fully sufficient when the telescoj^e is always to be 

 used by the same person ; but when difix^rent persons are to use it, then the dia- 

 phragm, which supports the micrometer, must be constructed so as to be easily 

 moved backwards or forwards, though that motion needs not be greater than 

 about a 10th or an 8th of an inch. This is necessary, because the distance of 

 the focus of the same lens appears different to the eyes of different persons, and 

 therefore, whoever is going to use the telescope for the mensuration of any 



