INDUSTRIAL PROGRESS DURING THE YEAR 1875. lxiii 



means of a micrometer screw. The apparent size of the ob- 

 ject is measured from two stations on the same line with 

 this object, and by a simple calculation, knowing the dis- 

 tance between these stations and the focal length of the 

 telescope, the distance of the object is obtained. The error 

 in the measurements made does not exceed five per cent. 



Jacques has determined, in the laboratory of the Massa- 

 chusetts Institute of Technology, the percentage of light 

 transmitted through glass plates placed both perpendicu- 

 larly and obliquely to the ray. The plates were ordinary 

 window -glass carefully cleaned. The original light being 

 100, one plate transmitted 89.5 per cent., four plates 69.3 

 per cent., seven plates 55 per cent., and ten plates 45.3 per 

 cent. When the plates are oblique to the ray, the amount 

 transmitted by one plate decreases rapidly with the obliq- 

 uity, while with ten plates it actually increases until the ob- 

 liquity reaches 55. 



Cornu has proposed a very simple mode of correcting tel- 

 escopic object-glasses for photographic rays, by separating 

 more or less from each other the lenses composing them, an 

 idea originally suggested by Sir John Herschel for restoring 

 overcorrected objectives. Since the focal distance for chem- 

 ical rays is about one-half per cent, of the principal focal dis- 

 tance behind that for luminous rays, the necessary correction 

 is effected by separating the flint and crown components by 

 this amount, and then carefully adjusting. Cornu has used 

 the method with success on an object-glass of four inches' 

 aperture ; the method of Rutherford is, however, to be pre- 

 ferred for glasses much larger than this. 



Pickering and Williams have investigated the foci of lenses 

 placed obliquely, from which it appears that even the most 

 carefully corrected lenses may still be defective in this re- 

 spect. In a photographic camera, for lines passing through 

 the axis, the surface, instead of being plane, should have a 

 radius of curvature of only 0.3 the focus, while for lines per- 

 pendicular to these the curvature should be 0.7 the focus. 

 Curiously enough, the actual curvature in the normal eye is 

 about 0.5, or the mean of the above numbers. 



Kriiss has described a new eye-piece formed of a divergent 

 flint lens, placed between two convergent lenses of crown, so 

 that the faces in contact have the same radius of curvature, 



