324 



NA TURE 



VFcb. 3, 1887 



polar area of depression in tlie higher regions of the 

 atmosphere is shown by the movements of the higher 

 clouds. Any one who will be at the trouble to chart out 

 these phenomena will feel that the neat little orographical 

 maps of the atmosphere with which some of our popular 

 writers on weather would present us are exceedingly 

 different from the realities. 



The terms " col," ridge," " trough," &c., for a similar 

 reason, while assisting the popular imagination, perhaps 

 assist it in the wrong direction, and I would, though witli 

 much deference to better authorities, suggest that such 

 terms as " arm," " band," " belt," " extension," &c., might 

 be employed with a little more safety. To the terms 

 " deep," " depth," " high," " height," might not my own 

 respectable old words, " intense," "intensity," even now 

 be found preferable? and for the word "shallow" the 

 word '' slight " in many cases be advantageously substi- 

 tuted .' I am aware that in a magazine article or in a 

 weather report some variation of terms and expressions 

 is frequently desirable, but the cover has not yet been 

 fairly drawn ; and an abundance of useful words is still 

 available, without recourse being had to terms either 

 borrowed from foreign languages or expressive of incorrect 

 ideas. W. CLEMENT Ley 



NOTE ON INSTANTANEOUS SHUTTERS 



' P" HE introduction of rapid dry plates having made a 

 1 general demand for mechanical shutters, a large 

 variety are now offered for sale by the various makers. 

 Many of these shutters are neat and ingenious, but nearly 

 all have a tendency to shake the camera during exposure, 

 and in the only one which 1 have seen for sale in which 

 this mistake has been avoided the photographic efficiency 

 of the arrangement has been impaired by the opening 

 being made to assume the form of a gradually expanding 

 and contracting hole ; the idea being, I am told, that 

 while the opening is small it will act as a stop and secure 

 definition. This, of course, is true to a certain e-ttent — 

 how far, I will inquire presently. 



I do not know whether the general theory of mechanical 

 shutters has been discussed, but if it has it is certainly .lot 

 well known, and perhaps the following remarks, which 

 point out what the photographic efficiency of the various 

 classes of shutters is and their effect on the steadiness of 

 the camera, may be of some use. 



Shutters may be divided into two chief classes, viz. 

 those in which the principal moving part consists of a single 

 piece, and those where the moving parts are multiple ; 

 the great difference between them being that, while the 

 first class must exert either a force or a couple on the 

 camera during exposure, the second class may be so 

 designed as to exert neither. The first class con- 

 sists of drop-shutters and revolving disks with an 

 aperture which passes across the lens, and of those 

 shutters where a sliding piece rises and falls or a 

 hinged piece opens and shuts. Cf the second class I 

 only know of one as being in the market, though probably 

 many amateurs may, like myself, have made them for 

 their own use. In this shutter two plates, occupying the 

 position of the ordinary stop in the lens, separate and come 

 together again. Each plate has a deep V-shaped notch in 

 it ; the apex of each V when the shutter is closed being 

 in the axis of the lens. The opening is therefore a quadri- 

 lateral figure which gradually expands and contracts. 



The mechanical arrangements of nearly all the shutters, 

 except those belonging to the revolving disk and drop- 

 shutter class, are such as to make the motion of the 

 shutter a simple harmonic function, or nearly so, of the 

 time from the commencement of exposure, while in the 

 drop-shutters and disks the aperture may be taken as mov- 

 ing across the lens with a nearly uniform velocity. This, 

 of course, would not be true if the motion of the parts 



was quite free under the action of the driving force, but 

 friction enters largely into the account ; and even if it 

 did not, no large error will be introduced in calculating 

 the photographic effect of shutters of this class by assuming 

 that velocity of the moving part is uniform during exposure 

 and equal to its mean velocity. 



The photographic effect of a shutter is measured by the 

 sum of the products of each element of aperture brought 

 into action by the shutter and the time for which that 

 element acts. This measures the total amount of light 

 which passes through the lens during exposure, but it does 

 not necessarily follow that the light should be uniformly 

 distributed on the sensitive plate. This, indeed, only 

 happens when the shutter is at the optic centre of the 

 combination. 



In mathematical notation, if the path of a point in the 

 shutter be along a line x, and if U be the area of the 

 lens expressed in terms of .i-, and 7", the time for which 

 dU, the element of area exposed in passing from x to 

 .r + dx, acts, then the photographic effect of the shutter is 



/ T^dU, taken between the proper limits of .r. 



The photographic efficiency of a shutter may be taken 

 as the ratio of this quantity to the whole area of the lens, 

 multiplied by the whole time of exposure, or T'U'. 



The result of integrating the above expression may 

 always be put in the form 



aT'U', 

 where rt is a numerical constant, which therefore expresses 

 the efficiency of the particular shutter considered. 



I subjoin a few results showing the efficiency of several 

 different types of shutter: — 



(1) Drop-shutter with circular aperture (uni- 

 form velocity) 



(2) Harmonic opening from one side (.r pro- 

 portional to cos//) 



(3) Harmonic opening from centre, the open- 

 ing being a circular hole of radius p (proportional 

 to sin p/) 



(4) ' Harmonic opening from centre, the 

 aperture being formed by the edges of two plates 

 which recede from a diameter of the lens and 

 the boundary of the lens {x proportional to 

 smp/) 



It will be seen that as far as efficiency goes the drop- 

 shutter is lowest on the list. 



The next two have the same efficiency, but while the 

 second has a tendency to shake the camera the third has 

 not. If, instead of assuming that the aperture in this 

 case was circula%we had made it square, as in the shutter 

 before referred to, the efficiency would not have been quite 

 as great as "5. 



No. 4 has the highest efficiency of any, viz. 764, and 

 differs from the last merely in having no V-shaped notches 

 in the plates which close the aperture, so that the opening 

 begins as a slit instead of a point. Thus by the adoption 

 of the square expanding aperture nearly 40 per cent, of 

 possible efficiency is lost. 



The gain in definition caused by the aperture acting as 

 a stop may be estimated by comparing the amount of 

 light (Zj) admitted while the opening is small enough to 

 make the definition good, with the total amount of light 

 admitted (Z.) minus (Z,j), remembering that the greater the 

 aperture up to which the shutter may open without sensibly 

 impairing the definition the less is the possible gain in 

 definition from the use of a stop. Thus, suppose the 

 greatest aperture consistent with good definition to be 

 /)"' X full aperture [R'). Then the use of a stop of radius p 

 can only reduce the radius of the circle of confusion about 



the image of a point by — ^— ^ times what it would have 



' This is the form which I use, but I am not .iware of any shutter of the 



•5 



a = 764 



