Jtjne 18, 1909] 



SCIENCE 



963 



to other optical questions. The various cir- 

 culars which issued from the The Scientific 

 Shop were always his own, and those who have 

 had the pleasure of reading them will recog- 

 nize that they are unique in accurate scholar- 

 ship, in instructiveness and in absolute candor. 

 No account, however brief, of Mr. Porter's 

 work would be fair, not to say adequate, which 

 did not include some reference to his several 

 large volumes of manuscript " notes," in 

 which he was in the habit of recording new 

 ideas and experimental results. A single 

 citation which will serve to illustrate the 

 cleverness, ingenuity and skill of the man may 

 also be of value to makers of lenses. It is a 

 note, referring to an extension of the bene- 

 fits of the bifocal lens invented by Benjamin 

 Franklin and having interest as being the 

 last problem upon which he was engaged, and 

 is as follows: 



MITLTIFOCAL SPECTACLE LENS 



Intended to replace the bifocal lens. Would 

 have two advantages ( 1 ) " invisibility " in the 

 sense of not being evidently different from a single 

 focus lens and (2) the multifocal property should 

 give clear vision at any distance from (say) 10 

 inches to infinity. 



One face of the lens is curved only in the hori- 

 zontal plane and the other only in the vertical 

 plane, the radii of each surface growing shorter 

 from top to bottom of the lens in about the same 

 ratio. Thus one surface may be a circular cone 

 and the other a spiral cylinder. The curvatures 

 need not increase uniformly from top to bottom, 

 but the increase should be at the same rate on 

 each side so as not to introduce cylindrical error. 

 ■Vertical or horizontal astigmatism can be cor- 

 rected by making one surface of uniformly greater 

 curvature than the other. The surface may be 



mechanically ground and polished by simple mech- 

 anism. Angular astigmatism can possibly be cor- 

 rected by a superposition of curves, but the sur- 

 face would not then be " ruled," and the polishing 

 operation might be less certain. 



The fault of the lens is that the curvatures and 

 foci are different at top and bottom of the eye. 

 This difference is, however, slight in a large lens, 

 and unless presbyopia is great may not be enough 

 to cause inconvenience. 



A. B. P. 



August 17, 1907 



Under date of January 24, 1909, occurs a 

 two-page note describing in detail a method by 

 which the grinding tools for such a multi- 

 focal lens may be made. Then the following! 



MULTIFOCAL LENSES 't 



Could be made with spherical surfaces by using 

 a glass varying in density (refractive index) from 

 top to bottom. Such a glass could be made by 

 using a flat melting pot heated from above (to 

 avoid convection currents) in which was placed 

 in very thin layers the varying materials needed 

 to give the varying density. A " guard ring " 

 should be put in the center of the melting pot to 

 avoid convection currents due to unequal heating 

 or cooling around the sides of the pot. The glass 

 should be kept in a molten condition long enough 

 for diffusion to make the density gradient uni- 

 form, and the heat should be turned off so slowly 

 that the top always remains hotter than the bot- 

 tom to avoid convection. A. B. P. 



February 5, 1909 



Hundreds of suggestive " notes " of this 

 type lead one to wish that it might have been 

 possible for a mind so fertile in resources to 

 have devoted its energies to investigation and 

 pure science, unhampered by the daily routine 

 of teaching or commerce. 



Henry Crevf 



BANQUET IN HONOR OF PROFESSOR 



BESSEY 



The Botanical Seminar of the University of 

 Nebraska gave on June 5 an anniversary ban- 

 quet in honor of Charles Edwin Bessey, to 

 celebrate the twenty-fifth anniversary of his 

 professorship in Nebraska, preceded by four- 

 teen years of professorial service at the Iowa 

 Agricultural College. Dr. Roscoe Pound pre- 



