USEFUL TO THE MICROSCOPIST 1131 



Formula relating to Prisms 



Lot i == the refracting angle of the prism, $ the angle of incidence on 

 the first surface, <' the angle of refraction at the first surface, ^ the angle 

 of incidence in the prism at the second surface, and \// the angle of re- 

 fraction on emergence ; then the total deviation 



When the ray passes through the prism symmetricaUy the deviation 

 is at a minimum : < = >//, <j>' = \js -, and 



sinl 



by which formula the refractive indices of media can be found, because 

 both i and D are capable of accurate measurement. 



Formula relating to Conic Sections 

 Ellipse. Let A = major axis ; a = minor axis. Then 



Focus = A-N/A 2 -^ ........ (lh) 



Parabola. Let A = height; a= - base. Then 



2 



Focus = -^- ......... (lv) 



4: A. 



Hyperbola. Let A = major axis ; a minor axis. Then 



Focus = Vg ^- A ....... (Ivi) 



Works consulted: Coddington, Camb. 1830; Parkinson, Camb.; 'Ency- 

 clopaedia Britannica' ; ' Journal E.M.S. ' ; Heath, Camb. 1887, &c. It will be 

 seen that several of the formulse have been entirely reset, while some 

 appear now for the first time. 



