1814.] A Treatise on New Philosophical Inslrnments. 457 



In Older to convey to the reader some idea of the instruments and 

 methods described in this volume, and of the experimental results 

 contained in the fourth and fifth books, we shall direct his attention 

 to those which appear to us of most importance, and endeavour to 

 give as intelligible an account of them as can be done without the 

 aid of figures. 



Tlie various micrometers which have hitherto been used may be 

 divided into tuo kinds, viz. those in which one image of the object 

 is formed, and those in which two images are formed. The single 

 image, or tJie wire micrometer, consists of two parallel wires, which 

 are made to approach to, or to recede from, each. other, by the 

 revohition of a line screw, containing 50 or 100 threads in an inch. 

 These wires are separated till they exactly comprehend the object to 

 be measured, and the number of revolutions of the screw wliich are 

 required to bring tbe wires together becomes a measure of the angle 

 subtended by the object, the value of one or more revolutions having 

 been previously determined by experiment, Tiiis instrument in its 

 best state is not only very expensive, but is extremely complicated, 

 easily deranged, and liable to numerous sources of error, which 

 liave been pointed out by Bradley, Herschel, and other astronoiners. 



The principle of the new niicroraeter invented by Dr. Brewster 

 is to have one or more pieces of wires absolutely fixed in the field of 

 the telescope, and to separate them by an optical instead of a 

 onechnnicul contrivance ; for it is obviously the same thing whether 

 the wires are opened to embrace the sun's diameter, or the sun's 

 diameter magnified till it fills the space between tiie wires. This 

 change, however, upon the magnitude of the object must be effected 

 in a part of the telescope anterior to the wires. In order to accom- 

 plish this, a second object glass is made to move between the prin- 

 cipal object glass and its focus, by which means the magnifying 

 power of the instrument, and consequently the angle subtended by 

 the wires, may be constantly changed. When the object glasses are 

 in contact, the angle subtended by tlie wires is a maximum ; and 

 when they are at their greatest distance the angle is a minimum, 

 and every intermediate angle between these two is measured by a 

 scale oj ecjiial parts cfp/al to the focal length of the principal object 

 glass. In tbis construction the imperfections of tbe screw, the 

 error arising from the uncertainty of liic zero, from the bad center- 

 ing of the lenses, from the want of paraljelisin in the wires, and 

 from the minuteness of the scale, are completely removed. 



The j)rinciple of tlic preceding micrometer applies most happily 

 to the (iregorian and the Cassegrainian reflecting telescopes ; and, 

 what at first sight may ajjpear paradoxical, these instruments may 

 be converted into a very accurate micrometer, almost without tlie 

 aid of any' additional apparatus. A moveable object glass is not 

 necessary, as in the former case, for the magnifying power of these 

 reflecting telescopes may be varied merely by varying the distance 

 between the eye-piece and the great speculuni. 



The same optical principle constitute! tlic foundation of the neiu 



