I. DISTRIBUTORS. 173 



SECTION 7. LIGHT. 



WEST GALLERY, UPPER FLOOR, ROOM f x Q . 



DETERMINATION VELOCITY. 



791. Original Apparatus, by M. Fizeau, for measuring 

 the velocity of light. Polytechnic School, Paris. 



792. Apparatus, by M. Foucault, for measuring the velocity 

 of light. Polytechnic School, Paris. 



I. DISTRIBUTORS. 

 . LENSES. 



795. The Phakometer (Snellen), for the determination of 

 the power of lenses (by the method of placing object and image 

 at equal distances from the lens). Dr. Snellen, Utrecht. 



The object (points of light on ground glass) and the screen upon which 

 the image is received are moved in a perfectly similar manner, but in opposite 

 directions, each by an enclosed steel spring. 



For the determination of weak lenses, an auxiliary lens, No. 275, is placed 

 ou each side of the lens examined (at a distance of 0-025 from the centre). 



The screen which receives the image moves alongside a scale, upon which 

 at each double focal distance of the system of the three lenses (obtained by 

 calculation) is marked the power of the lenses used in ophthalmological 

 practice. Within -$ " dioptric " one can with sufficient accuracy estimate 

 how much the lens examined differs from the powers marked on the scale. 



The image having constantly the same magnitude, precise adjustment is 

 easy. The screen possesses a diagram of the image. The image of the 

 points of light covering this diagram, the centre of the glass coincides with 

 the diameter of the instrument. Not only the focus, but also the centre can 

 be determined directly. 



The scale may be controlled at any time by determining the strong glasse* 

 directly, the auxiliary lenses being removed. Then the double-focal distance 

 is to be taken from the corresponding principal plane to the point where the 

 image is formed. 



The instrument as yet is only adapted for symmetrical (biconvex) lenses. In 

 order to determine plano-convex or periscopic glasses, it will be best to place 

 two glasses of equal power one against the other, so as to obtain a symmetrical 

 form. According to the calculated principal planes of this system, a scale 

 may be computed. 



