Mr. Robert Hunt on Chromo-Cy allotype. 435 



A d x u 4- Bd v u + C d z u = 0. 

 Now the equation to the osculating plane at the point (xyz) is 



A(X-*) + B(Y- i jO+C(Z-*) s =0; . . (9.) 

 and the equations to the normal are 



(10.) 



d„ u d y u d z u 



X-x~Y-y Z-z' ' * 

 X, Y, Z being the current coordinates of a point in the plane 

 or normal; and when the plane (9.) contains the normal (10.), 

 we have the condition 



A d u + BdgU +- C d z u = 0. 

 Hence equation (8.) expresses that the osculating plane con- 

 tains the normal ; now this is the property of the shortest line 

 between two points on the surface. Also we have 

 d s T = — (Xd s x + Yd s y + Z d s z) = 0, 

 .*. T = constant = /<•, 



k 

 and pressure = — on an unit of length. 



Since the tension is constant, we must suppose the two ex- 

 tremities either to be fastened to the surface or pulled by any 

 equal forces; and then the curve into which the string will be 

 arranged will coincide with the shortest line that can be drawn 

 on the surface between those points. 



I remain, Gentlemen, 

 St. John's College, Cambridge, Your obedient Servant, 



March 25, 1844. J. A. CoOMBE. 



LXIII. Chromo-Cyanotype, a neiv Photographic Process. By 

 Robert Hunt, Secretary to the Royal Cornwall Poly- 

 technic Society. 



To Richard Taylor, Esq. 

 Dear Sir, 

 EING engaged in the investigation of some changes pro- 

 duced by the agency of the solar rays upon various me- 

 tallic solutions, I was led to notice some very remarkable 

 effects connected with the precipitation of the per- and proto- 

 salts of iron. It would be premature to publish these expe- 

 riments, but there has arisen out of them a photographic pro- 

 cess of so pleasing a character, affording phaenomena of so 

 curious a nature, and the process is so exceedingly simple, 

 that I am induced to forward a description of it to you, not 

 doubting but it will be interesting to the readers of the Philo- 

 sophical Magazine. 



To distinguish this from the cyanotype processes of Sir 



2 F2 



B 



