1814.] A Treatise on New Philosophical Imirvments. 459 



intersect one another within the circle or without it, the vahie of 

 the angle is easily obtained ; and by dividing the circular scale into 

 180° instead of 360°, and fixing the zero of the scale in a particuiar. 

 manner, the angle is read off with the very same facility as if the 

 wires had crossed each other in the centre of the field. 



The other instruments described in this chapter are a double 

 image goniometer, a diagonal telescope, and neiv protractors for 

 measuring angles. 



The instruments described in Book III. are chiefly the applica- 

 tions of the micrometers in Book I. to military and naval telescopes 

 for measuring angles and distances, with the methods of construct- 

 ing the scales, and rules for using the instruments. Tiiese instru- 

 ments are, 



1. A micrometrical telescope for measuring distances. 



2. A double image telescope and coming- up glass, for measuring 

 distances at sea. 



3. A luminous image telescope for measuring angles and distances 

 during night. 



4. Instruments for measuring inaccessible distances at one station. 



5. Optical instruments for measuring inaccessible distances at one 

 station. 



The Fourth Book is principally devoted to the description of 

 instruments, and methods emj)li)yed in a very extensive series of 

 experiments on light and colours. 



In the second chapter the author has described a new method of 

 measuring the refractive powers of solid and fluid bodies, and has 

 given copious tables of the refractive powers wbich he has by this 

 means ascertained. The method consists in introducing a portion 

 of the substance between the object glass of a compound microscope 

 and a plate of parallel glass. By this means a concave lens of the 

 substance is formed : the focal length of the object glass is dimi- 

 nished ; and in order to obtain distinct vision, it is necessary to place 

 the object at a greater distance from the object glass. The distance 

 therefore of the object fiom the object glass, when distinct vision is 

 procured, becomes a measure of the refractive power. By this 

 method the author obtained the most transparent lenses of aloes, 

 pitch, opium, bird-lime, asafoetida, dragon's blood, caoutchouc, and 

 many other substances through which light had never befoic been 

 regularly refracted. 



The tables of refractive powers that had hitherto been publislied 

 never contained more than G'O or 70 substances, hut those which 

 are given in the present work contain about 300 nH-a^ures of 

 refractive powers. Many of these results are hifibly interesting to 

 the chemist and the natural philosopher: and in the Annals of Flii- 

 lou>fj/n/, vol. ii. p. 30J, wc have already had occasion to notice some 

 of the most important. This chapter also contains tlie accounl of a 

 mctiiod of measuring tiie refractive power of solid fr.tgmenls. 



'i'hi' third chapter is occupied with the description of a new 

 iostruiutut for measuring the dispersive poutis of solid and fluid 



