N O T E S F R O M T H E L A B O R A T O R Y OF P U R E S C I E N C E , N E L A R E S E A R C H L A B O R A T O R I E S . *

THE RELATION BETWEEN FIELD BRIGHTNESS AND THE SPEED OF RETINAL IMPRESSION.

B y P e r c y W. Cobb.

THE time values ( t) of the table represent the times of exposure of a black circular dot in the centre of a white screen,

F m t

.100

.075

.150 I I

I t I

. IZ5 I

I

I

I

I I I

I

I

I I I

050 ~ / ~

/ .OZ5

i I I

I I

i I

I I

I I

, I 1 ]o

I I

I I

i I ~ ~.

/ t

I I i

I I

I i I I

I0 20 50 C. p e r Scl.PI.

/ i

t I

i

I

I

I

I I i

i

i

I

IOO zoo

~1 ~Reciprocal of weighted mean threshold time, as given in Table I, x/t',plotted against the.%g- ari thm of the brightness. Curve I, seven subjects. Curves I I , for subjects A, I , and S in high (a) and low (b) ranges of brightness as in Table I.

solid lines, "l ight" series (L). . . . . . . . . broken lines, "da rk" series (D).

necessary in order that the subject may correctly state whether or not the dot has been actually shown, the purpose of the work being'to show how these time values change with changes in the

* Communicated by the Director.

855

856 NOTES FROI~I N E L A RESEARCH LABORATORIES. [J. F. I.

illumination. A detailed description of the apparatus used to produce such a test stimulus appears in the original and in prior communications therein cited. The experiments were conducted with seven subjects over a fairly high range of illuminations with the results (t) given in the table. In the figure are plotted the reciprocals of these time values as given in the right-hand section of the table, ! / t . Three of the subjects were investigated over a lower range of illuminations, their results for the two ranges being given in sections a and b of the table, respectively, and being plotted in curves IIa and IIb, Fig. I.

The subjects varied rather widely in the absolute minimum

TABLE I.

t x/t C. per MI. Sq. M. L I D I M L ] D I M

(a) Subjects A, I, and S

342 IO7 6.79 6.82 6.8o .147 .148 .I48 2Ol 63.I 7.o8 7.38 7.23 .141 .I36 .138 Io5 33.0 8.7I 8.6I 8.66 . II 5 . I I6 .I16

51.5 16.2 9.95 IO.O5 IO.O .IOO .0996 .0998 24.8 7.8 I3.4 I3.2 13.3 .0748 .0760 .0754

(b)

31.3 8.83 11.5 11.2 11.4 .0867 .0889 .0878 I8.9 5.95 13.3 I3.5 13.4 .075o .o74I .0746

7.73 2.43 20.9 21.5 21.2 .0478 .0466 .0472 3.33 I.O5 34.0 33-3 33.6 .0294 .o3oi .0298

Seven Subjects

342 io 7 8. 9 8.8 8.8 .112 .II3 .112 2Ol 63.1 9.4 9.9 9.6 .lO7 .iOl .lO4 I05 33.0 I I . I 10. 9 II.O .0904 .0916 .o910 51.6 I6.2 I2. 9 12.8 12.8 .0776 .0782 .0779 24.8 7.8 16. 9 16.8 I6.8 .0592 .0594 .0593

time required. The average times for all light conditions com- pared varied from 7-3 to 28. 7 8 for the several subjects. The difference between the L and D values (see explanation of table) is not more than is probable as chance variation.

The geometric mean time of darkening of a circular spot 4.24 mm. in diameter, at 6.or metres distance, necessary in order that a threshold impression may result.

The sub-heading L means extensive bright surroundings (46.6x 47.6 ° in the visual field); D, bright surroundings restricted (5.8 x 5.2°), the remainder being da rk ;M, the mean of the L and D values.

June. v)23.] NOTES FROM N E L A RESEARCH LABORATORIES. 857

Brightness is stated in candles per square metres and in milli- lamberts ; and the fields indicated by L and D were kept as nearly as possible uniform with the area about the stimulus.

The times ( t) are stated in thousandths of a second (8). The reciprocals of these ( I / t ) are plotted in Fig. I.

A N I N T E R C O M P A R I S O N OF T H E HIGH T E M P E R A T U R E SCALES IN USE IN T H I S COUNTRY W I T H T H O S E

IN USE IN ENGLAND.

By W. E. F o r s y t h e .

SEVERAL years ago Nela Research Laboratory sought an inter- comparison of its high temperature scale with those of the Bureau of Standards, the Physical Laboratory of the University of Wis- consin and the Research Laboratory of the General Electric Corn-

TABLE I.

Intercomparison of Temperature Scales of National Physical Laboratory, Bureau of Standards, and Nela Research Laboratory.

(c2=I4,35ou X deg.: X---o.665u: melting point of Au.=1336° K. arid melting point of Pd. = I828 ° K.).

Lamp,

T - - 7 7 - - B

T - - 7 8 - - C

II . 7

15.4 20.4 27.4

N. R. L. Current. April, '2o.

5.2 14IO ° K. 6.7 1599 8.5 1796

2106

!I825 2262

12746

N. P. L. April, '22.

I4o5°K. I597 I794 21o6

1825 2266

12758

N.R.L. June. '22.

I4o6°K. I595 1794 2104

1826 2266 2753

B. of S. Jan., ' 2 a . I

14oi ° K. [ 1596 1792 21o5

1824 2265

2755

N. R. L. Jan., '23.

14oI°K. I591 179 I 21o4

I828 2269 2755

pany at Schenectady through measurements in each of the laboratories on several tungsten filament lamps specially prepared in this laboratory. The results obtained, as presented before this Society in 1917,1 showed a very good agreement. Since that time the intercomparison has been extended to include the National Physical Laboratory of London through the courtesy of Sir J. E. Petavel. Doctor Stratton of the Bureau of Standards kindly agreed also to have measurements made on the same lamps, and the results obtained in those two laboratories, together with the values found in Nela Research Laboratory, are given in Table I.

1Phys. Rev., 2 S II, p. t39 (1918). \ToL, I95--No. 117o--59