Ogden Nicholas Rood. 75 



a kind of temporary cramp similar to that produced by 

 electric shocks : made many experiments upon the best forms 

 for elongated projectiles, and demonstrated the superiority of 

 American rilled firearms, and American marksmanship; 

 devised a method of producing stereographic pictures by 

 hand ; discussed Dove's theory of luster, and devised many 

 new experiments for producing it ; studied the practical appli- 

 cation of photography to the microscope, and showed how it 

 might be used to determine the character of the minute mark- 

 ings upon diatoms. 



The years from 1864 to 1869, after his settlement at 

 Columbia College, were comparatively uneventful so far as 

 publication was concerned, doubtless owing to the great 

 demands of his new position. But, shortly after, a series of valu- 

 able researches was begun, and, throughout the rest of his life, 

 contributions appeared' in rapid succession. During this period 

 he investigated the nature and duration of the discharge of a 

 Leyden jar connected with an induction coil, and determined 

 with an accuracy hitherto unattained the duration of the bright 

 spark from a jar of small capacity. This work was resumed 

 later, and he showed that the bright spark lasted for a period 

 not longer than forty billionths of a second. He also, in con- 

 nection with this research, studied the question of the amount of 

 time of illumination necessary to vision, finding that a duration 

 of four billionths of a second was sufficient for distinct vision. 

 The experience gained in these experiments led him to study 

 the character of lightning, using for the purpose a rotating 

 card-board disk, which in his hands proved an efficient instru- 

 ment of measurement in many applications. He thus, for the 

 first time, determined the duration of the flashes, finding an 

 average value of the five hundredth part of a second, but sub- 

 ject to great variations. He also showed the multiple character 

 of the flashes, and that they often terminated in a single 

 isolated discharge of so short duration as to be, in comparison 

 with the others, practically instantaneous. In this period 

 also were carried out the experiments with the horizontal 

 pendulum, and the Sprengel pump, already referred to, and 

 later the work upon the measurement of high resistances. 



He had at intervals been much interested in the study of 

 color, and made many experiments in the measurement of the 

 intensities of colored lights. His first efforts in this direction 

 were made by the help of the rotating disk, in a comparison of 

 the gray from the black and white sectors of variable propor- 

 tion, so adjusted as to have the same apparent brightness as 

 the object studied. His skill in experimenting enabled him 

 to make consistent observations, and he obtained valuable 

 quantitative results. This subject was a favorite one with him, 



