844 



SCIENCE. 



[N. S. Vol. XX. No. 520. 



wisest men, disregarding at this juncture mere 

 scientific claims. The council should select 

 for presidents at this time not representatives 

 of science as merely to do honor but men of 

 large experience and sympathy with the asso- 

 ciation affairs. It is of less consequence what 

 the public thinks about the association than 

 what we shall do for ourselves. 



Realizing the gravity of the condition at 

 this critical time the council _will make wise 

 decisions only after full discussions in a gen- 

 erous spirit; and the membership should in 

 patience trust the collective wisdom of the 

 council. Herman L. Fairchild. 



Rochester, N. Y., 

 December,^ 1904. 



• ' THE PROBLEMS OF EXPERIMENTAL PSYCHOLOGY.' 



To THE Editor of Science: On p. 788 of 

 your issue of December 9 (second column, 

 line 8), I am made to speak of ' classification 

 a posteriori.' What I wrote, what the sense 

 requires, what I saw in proof, and what I left 

 in proof, was ' classification a potiori.' On 

 p.* 794 (bottom of first and top of second 

 columns), I am made to say : ' we analyse and 

 trace to their conditions total consciousness.' 

 What I wrote, what grammar requires, what 

 I saw in proof, and what I left in proof, was 

 ' consciousnesses.' A little knowledge, even 

 in a proof-reader, is a dangerous thing. 



E. B. Titohener. 



COENELL UnI\'EESITY, 



December 10, 1904. 



[The errors probably would not have oc- 

 curred if Professor Titchener had returned 

 his proof to the editor in accordance with the 

 instructions accompanying it. It was sent 

 directly to the printers. — Ed.] 



SPECIAL ARTICLES. 



A suggestion looking towards ultra-micro- 

 scopy. 

 The visibility of an object both to ordinary 

 vision and when helped by telescope or micro- 

 scope depends upon a favorable combination 

 of several physical conditions. (1) The ob- 

 ject must send us ethereal waves whose lengths 

 lie between the limits of O.-38-and 0.76 microns 



or the violet and red ends of the spectrum 

 respectively. (2) The difference between the 

 intensity or color of these waves and those 

 coming from the adjacent background must 

 be appreciable to our nervous system. (3) 

 The focus on the retina must be sharp. (4) 

 The duration of the image on the retina, or, 

 as the photographer would say, the length of 

 the exposure, must be long enough to enable 

 the brain to appreciate the details of the 

 image. 



By means of photography we are able to 

 make long exposures and the fourth condition 

 can be satisfied to such extent that fleeting 

 pictures are caught by instantaneous expos- 

 ures, while the faintest nebulae and stars are 

 caught by exposures that last many hours. 

 Becquerel's first photograph by the rays that 

 are called after his name was by an accidental 

 exposure of many days. 



By means of the schleier method, originally 

 due to Foucault, we can overcome the diffi- 

 culties of the second condition and photo- 

 graph moving air waves when properly illu- 

 minated, and this method can be applied to 

 microscopic objects and liquid substances as 

 well as to the larger motions of the air that 

 have been photographed by Mach, DuBois and 

 others. 



The ultimate limit of visibility is also de- 

 fined by the second condition or the wave- 

 length and intensity of the illuminating light 

 that can affect the retina, or the sensitized 

 photographic plate. An object that is visible 

 by monochromatic violet light may not be 

 visible by monochromatic red light or vice 

 versa, just as a body that can vibrate to a 

 given high pitch is often too small to send 

 out a low note. An ear that is too dull to 

 hear the low notes may hear a high pitch. 

 Our retina is so constructed as to be insensible 

 to ultra-violet light, but we can by fluores- 

 cence make short waves become visible, i. e., 

 an object illuminated by ultra-violet light 

 whose wave-lengths may be anywhere from 

 zero to 0.38 microns may be too small to be 

 directly affected by long waves, but will, by 

 fluorescence convert the short waves into 

 longer ones whose lengths may be any given 

 multiple of the ultra-violet wave, and will, 



