HARDENING OF STEEL. 



31 



that appeared to give the most perfect distinctness was about 

 one-fifth part of the focal length in diameter j and when such 

 an aperture i9 well centered, the visible field is at least as 

 much as twenty degrees in diameter. It is true^ that a portion 

 of light is lost by doubling the number of surfaces ; but this is 

 more than compensated by the greater aperture, which, under 

 these circumstances, is compatible with distinct vision. 



Beside the foregoing instances of the adaptation of peri- pe j f^op^c 11 ° 

 scopic principles, I should not omit to notice their application principles to 

 to the camera lucida ; as there ii one variety in its form, that f U g i( J* mera 

 was not noticed in the description which I originally gave of 

 that instrument*. 



In drawing, by means of the camera lucida, distant objects 

 are seen by rays twice reflected (d, fig. 5), at the same time 

 and in the same direction that rays (e) are received from the 

 paper and pencil by the naked eye. The two reflections are 

 effected in the interior of a four-sided glass prism, at two pos- 

 terior surfaces inclined to each other at an angle of 135 de- 

 grees. In the construction formerly described, the two other 

 surfaces of the prism are both plane, through which the rays 

 ar« simply transmitted at their entrance and exit. But since 

 an eye that is adjusted for seeing the paper and pencil, which 

 are at a short distance, cannot see more distant objects dis- 

 tinctly without the use of a concave glass, it may be assisted 

 in that respect by a due degree of concavity given to either, 

 or to both the transmitting surfaces of the prism. It is, how- 

 ever, to the upper surface alone that this concavity is given 5 

 for since the eye is then situated on the side toward the centre 

 of curvature, it receives all the benefit that is proposed from 

 the pei iscopic principles. 



VI. 



Practical Experiments on hardening Steel. By Mr E.Lydiatt, 

 Lecturer on metallurgy, and the mechanic Arts, &c. In a 

 letter from the Author. • 



To W. Nicholson, Esq. 

 Sir, 



THE desire I feel to be instrumental in promoting the cause Introduction, 

 of science and truth, makes me regret that indispensable avo- 



• Nicholson's Journal, XVII, p. 1. Phil. Magaz. XXVII, p S43. 



cations 



