On Mieroseopical Appliances. 79 



On the lines A, B, innumerable questions of refraction and 

 internal reflexion are answered on inspection. 



It gives at sight also in most cases to three places of decimals 

 the sines of angles, thus : — 



Sin. 21 J° = -365.* Sin. 10° 20' = 0-179. Sin. 5° 5' = 0-0885. 



Again, if a ray of light passes from water into diamond, wafer 

 y. = 1'S3Q diamond p = 2 "47, by placing these indices over each 

 other, at sight we see the following angles : — 



The last, 33°, being the angle of total internal reflexion, and 

 subtracting one from the other, the deviations at given angles of 

 incidence are at once shown on the rule. The rule thus shows 

 these quantities : — 



VI. — On Microscopical Appliances. By De. Eoyston-Pigott. 



No. I. 

 On a Useful Form of Bynameter Micrometer — the Kratometer. 



It some time since occurred to the writer, that an instrument could 

 be so contrived in the form of an eye-piece which should serve some 

 very uselul purposes of measurement, such as, (without the constant 

 use of the rule of three) 



(1) To give, at sight, the magnifying power of any objective 

 for any length of tube in use at the time of observation. 



(2) To ascertain the standard focal length of object-glasses of 

 aU kinds, whether conventional or solar (for parallel rays). 



(3) To ascertain the focal length of any given single lens re- 

 quired to be tested, 



(4) To deduce the actual focal length of eye-pieces. 



(5) To measure objects in the field of view under any power or 

 length of tube. 



(I) In the present number of the Journal I shall only embrace 

 the opportunity of describing the mode of fitting up the instrument, 



* The rule is made by Gary, 181, Strand. 



