236 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



lenses represented by the usual formulas may be given graph- 

 ically, thus facilitating materially the calculation of these 

 values. Upon two perpendiculars erected at the extremities 

 of a base-line are laid off equal spaces from this line, repre- 

 senting the principal focus. Diagonal lines drawn from the 

 corners to each of these divisions represent the conjugate 

 foci. It may be easily shown geometrically that these arc 

 in the ratio required. Direct measurement gives the value 

 of any one of these quantities when the other two are previ- 

 ously known. 



The same physicist has proposed a change in the manner 

 of numbering glasses for spectacles. They are now number- 

 ed in terms of the radius of curvature, expressed in inches, 

 the sign being plus or minus, according as the glass is con- 

 vex or concave. The new method proposes to number them 

 in terms of a new unit called a dioptric, which is the power 

 of a convergent lens of one meter in focus. Since the power 

 of a lens varies in the inverse ratio of the focal distance, the 

 number of any lens in the new system is easily obtained by 

 dividing one meter by the focal length of the lens expressed 

 in meters and fractions of the meter. These two systems 

 have a simple relation to each other. 



Javal lias described an apparatus for determining astigma- 

 tism, and at the same time the number and position of the 

 axis of the correcting glass regarded as a cylindrical lens. 

 Two vertical disks, movable around the same horizontal axis, 

 carry each a system of lenses, those of the first disk being 

 cylindrical, inserted in mountings toothed upon their borders 

 and irearino; with a toothed wheel so that the\ r can be simul- 

 taneously revolved ; those of the second disk are spherical. 

 The first disk is used to determine astigmatism by viewing 

 through its lenses a circle divided into sectors of 15 by ra- 

 dii; and having determined its direction and adjusted the 

 axes of the lenses to it, rotation of the disk gives the focal 

 adjustment sought. The second disk permits the myopia 

 and hypermetropia to be corrected. The instrument is called 



an optometer. 



2. Dispersion. 



Glan has contrived a new photometer for comparing the 

 intensities of lights which differ in color. It is composed of 

 a collimator carrying two slits, placed the one above the oth- 



