50 



NA TURE 



[yuly 13, 1882 



pearance of an almost circular disc elongated before and 

 behind by only a slight blur. 



It may be said, how is it possible to give such brief 

 exposures as the above ? I see no difficulty at all in the 

 matter. Let us take two examples, (1) of quick move- 

 ment, and (2) of very quick, but by no means the quickest 

 possible, movement. As regards the former, I can flip 

 with my forefinger, and with the greatest ease, a light 

 weight (such as a very small stone) nine feet up in the 

 air ; now the maximum velocity of the tip of my fore- 

 finger is that of the initial velocity of the stone, which is 

 calculated at once by the usual formula, v = >J 2 fs, or 

 taking 2 /= 64, which it is very nearly, v = 8 <J s, the 

 units being in feet and seconds. The velocity in ques- 

 tion is therefore 24 feet, or 288 inches per second. As 

 regards a very rapid movement, we may take that of 

 the wing of a bird, which can undoubtedly be rivalled 

 mechanically. A pigeon is by no means the swiftest of 

 birds, but it can fly easily at the rate of 35 miles an hour, 

 and the part of the wing by which it is chiefly propelled 

 and which cannot be its extreme tip, must move much 

 more rapidly than this ; let us say, very moderately, at 

 70 miles an hour, or 1,232 inches per second. 



Now the duration of an exposure depends on three 

 data, namely, the rapidity with which the screen moves 

 past the eye, the width of the slit through which the 

 momentary glimpse is obtained, and the diameter of the 

 available portion of the pupil of the eye. I prefer not to 

 limit the pupil by using a small eyehole which is a source 

 of much trouble in actual work, but to have as large an 

 eyehole as is in any way desirable. I find the width of the 

 pupil of my eye in an indoor light as measured by holding 

 a scale beside it and reading off in the looking-glass, to 

 be about 01 inch, and I use a slit of the same diameter. 

 The exposure begins when the advancing edge of the slit 

 is in front of the near edge of the pupil, and it ceases 

 when these conditions are reversed, in other words it lasts 

 during the time that the screen is occupied in moving 

 through one fifth of an inch. In the cases just taken of 

 velocities of 288 and 1,232 inches per second, the dura- 

 tion of the exposure would be the 1,440th and the 6,160th 

 part of a second, respectively. There is therefore no 

 difficulty either theoretical or practical about shortness of 

 exposure and sufficiency of illumination. The power 

 exists, and can be utilized, of seeing bodies in motion by 

 a rapid-view instrument, showing them in apparent still- 

 ness, and leaving a sharply-defined image on the eye, 

 that can be drawn from visual memory, which in some 

 persons is very accurate and tenacious. 



I find on trial that great rapidity of exposure is in no- 

 wise essential for analysing the attitudes of a galloping 

 horse or a flying crow. The instrument I commonly 

 carry with me is a very rude one, but convenient for the 

 pocket, and is shewn below. The duration of the ex- 

 posure given by it under the action of its spring is the 

 360th part of a second, but the beginning and end of the 

 exposure ought not to count, so little light passing 

 through the edges of the pupil at those times that what is 

 then seen is relatively faint and is disregarded. I esti- 

 mate its practical duration at about one 500th of a second, 

 and it is rather less when the finger acts with a sharp tap 

 in opposition to the spring. The instrument is shewn in 

 Fig. 1, without its sliding lid, which protects it from injury 

 in the pocket. A is an arm which turns through a small 

 angle round c, its motion being limited by two pins. Its 

 free end carries a vertical screen, R R, which is a cylindrical 

 (or better, a conical sheet described) round an axis pass- 

 ing through c perpendicular to the arm. As the arm 

 travels to and fro, this screen passes closely in front of 

 the end of the box, which is cut into a hollow cylinder (or 

 cone) to correspond. There is a slit in the middle of the 

 screen, and an eyehole in the centre of the end of the box. 

 When the slit passes in front of the eyehole, and the 

 nstrument is held as in Fig. 2, a view is obtained. A 



stud, S, projects upwards from the arm, and an india- 

 rubber band, B, passing round a fixed pin and a descending 

 spoke of the arm acts as a spring in causing the stud s to 

 rise through a hole in the side of the box, where the 

 finger can press it like the stop of a cornet A, piston. In 

 using the instrument it is held in the hand as in Fig. 2, 

 with the eyehole in front of the eye. Nothing is then 

 visible, but on pressing or tapping the stud the slit passes 

 rapidly in front of the eyehole, and the view is obtained. 



After this, the stud is released and the arm springs back 

 wards, when a second view can be obtained, or the eye 

 may be purposely closed for the moment. 



I measured the velocity of the instrument by filing a 

 nick on the stud and laying a light weight (a small bent 

 nail) upon it, after having temporarily put in a peg that 

 checked the arm in its recoil when the slit was opposite 

 the eyehole. Then holding the instrument firmly against 

 the wall with the projecting end of the stud as vertical as 

 might be, I drew back the arm and released it, and noted 

 the height to which the weight was tossed. It was three 

 inches. This gave the velocity of the stud in the central 

 portion of the arm, and from this datum the velocity of 

 the more distant screen was easily calculated. I have 

 made more elaborate instruments with multiple levers and 

 with revolving discs (Messrs. Tisley, 172, Brompton Road, 

 are now making one of these for me), but am not as yet 



prepared to recommend any one of them in particular. 

 Different sorts would be probably be wanted by different 

 persons. For instance it might in some cases be convenient 

 that the trigger should be pulled at the right moment by a 

 bystander, the eye of the observer being in the meantime 

 kept closely shaded from the surrounding light. Again, 

 there are periodic movements which would be best 

 analysed by a slit in a rotating disc whose period of rota- 



