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165 PHOTOGRAPHIC SHUTTERS.* . BY FRANCIS BLAKE. At the outset, I will say that the word “shutter” will be used throughout this paper, not because I deem it a correct name for the apparatus, but because more than thirty years of bad usage have so fixed it in the photo graphic vocabulary that it would be quixotic to try to displace it. Of course, “ opener ” would be quite as appropriate—or, rather, inappropriate—a name as “shut ter”; and, of course, “exposer” is the proper substitute for both, as its dictionary meaning defines precisely the function of the apparatus. The earliest mention of shutters with which I am familiar is in a most excellent dictionary of photography, edited by Thomas Sutton, and published at London in 1867. On page 156, under the heading of “ Instantaneous Shutters,” he says :—“ There are many methods of instantaneously admitting and shutting off the light from the sensitive plate. Mr. Wilson, who has been most successful in getting good pictures with rapid exposures, adroitly uses his Highland bonnet placed in front of the lens. Some use flap-shutters iu front of the lens; some a similar arrangement close behind the lens ; and Mr. England and others use a guillotine sort of shutter with a slot cut across it, which falls immediately in front of the sensitive plate. As the slot passes the plate, the parts thus exposed to light receive the full effect of the whole power of the lens. In some respects this is the best instantaneous shutter that has yet been devised; but it is apt to cause a vibration in the camera 'while iu the act of falling.” I have quoted the above because I feel sure that later on you will share with me surprise that the principle of the focal plane shutter, so clearly stated by Sutton, should have been entirely neglected by later workers in the photographic field. Before beginning original work, it was thought best to test the speeds of the best market shutters. This was done by means of an apparatus which I devised, and had the pleasure of exhibiting to the Boston Camera Club some years ago. The principle of the apparatus is simply photographing the image of the sun as reflected by a freely falling silvered ball, and deducing the time of exposure by applying a law of gravitation to the linear value of the distorted image. My apparatus consists of a vertical staff, about six feet in height, rigidly attached to an iron bed-plate. The staff is painted dead black, and is graduated downwards, on its front face, in white lines to feet and hundredths. At the top of staff is a movable piece readily adjusted to the height which brings the image of the sun, as seen upon the surface of the ball, exactly in line with the zero of the staff graduation. The silvered brass ball, 23 inches in diameter, and 2 lbs. 2 ozs. in weight, is suspended by a short piece of silk trant-line attached to a small vulcanite ring, which, in turn, is held by a spring-clip attached to the adjusting piece. On opening this spring-clip the ball is released, and, falling parallel to the graduated staff, is received into a padded box attached to the bed-plate. It is perhaps needless to say that the exposure is made while the ball is falling, and that the length of the exposure is computed from the scale readings of the beginning and end of the black line which marks on the negative the path of the reflected image of the sun. To facilitate the computation of results, I have pre * Paper read before the Boston Camera Club. pared the subjoined table, which gives the time of falling to the ten-thousandth part of a second for each hundredth of a foot from 1 '00 foot to 5’59 feet. The formula used in computing this table is that t = A/d when t = time; V g d = distance fallen; and g = 16'083 feet—the gravity constant at our latitude. The manner of using the table will be made apparent by the following example :—An exposure having been made with a Frosch duplex shutter, the beginning of the black line marking the course of the sun’s image on the negative was found to be opposite the 4'62 feet staff graduation, and its end opposite the 4'29 feet graduation. Referring to the table, it appears that— ft. s. Time of falling ... I... 4'02 = 0'5000 „ „ 4'29 = 0'5165 Time of exposure = 0'0165 With a well-made shutter the accordance of the results obtained with this apparatus is remarkable. For example, three consecutive tests for fastest speed of a Frosch duplex shutter gave the following values s. 1st test ... ... ... ... ... 0'0164 2nd 0-0167 3rd „ 0'0179 Mean 0'0170 Each test brought into play a different part of the staff graduation. TABLE GIVING THE TIME IN TEN THOUSANDTHS OF A SECOND FOR EACH HUNDREDTH OF A FOOT, FOR A FALLING BODY AT LATITUDE 42° 29. 3123456789 1 0 2494 2506 2518 2531 2543 2555 2567 2579 2592 2603 1-1 2615 2627 2639 2651 2663 2674 2686 2697 2709 2720 12 2732 2743 2754 2766 2777 2788 2799 2810 2821 2832 1 3 2843 2854 2865 2876 2887 2897 2908 2919 2929 2940 1 4 2951 2961 2972 2982 2992 3003 3013 3023 3034 3044 1'5 3054 3064 3074 3085 3095 3105 3115 3125 3135 3144 1 6 3154 3164 3174 3184 3193 3203 3213 3223 3232 3242 1-7 3251 3261 3270 3280 3289 3299 3308 3318 3327 3336 1 8 3346 3355 3364 3373 3383 3392 3401 3410 3419 3428 1-9 3437 3446 3455 3464 3473 3482 3491 3500 3509 3516 2 0 8527 3535 3544 3553 3562 3570 3579 3588 3596 3605 2'1 3614 3622 3631 3639 3648 3656 3665 3673 3682 3690 2’2 3699 3707 3716 3724 3732 3741 3749 3757 3765 3774 2 3 3782 3790 3798 3806 3815 3823 3831 3839 3847 3855 2 4 3863 3871 3879 3887 3895 3903 3911 3919 3927 3935 2-5 3943 3951 3959 3966 3974 3982 3990 3998 4005 4013 2 6 4021 4029 4036 4044 4052 4059 4067 4075 4082 4090 2-7 4098 4105 4113 4120 4128 4135 4143 4150 4158 4165 2 8 4173 4180 4188 4195 4202 4210 4217 4225 4232 4239 2 9 4247 4254 4261 4269 4276 4283 4290 4298 4305 4312 3 0 4319 4326 4334 4341 4348 4355 4362 4369 4376 4383 3-1 4391 4398 4405 4412 4419 4426 4433 4440 4447 4454 3-2 4461 4468 4475 4482 4489 4496 4502 4509 4516 4523 3-3 4530 4537 4544 4551 4557 4564 4571 4578 4585 4591 3-4 4598 4605 4612 4618 4625 4632 4639 4645 4652 4659 3-5 4665 4672 4679 4685 4692 4698 4705 4712 4718 4725 3'6 4731 4738 4745 4751 4758 4764 4771 4777 4784 4790 3'7 4797 4803 4810 4816 4823 4829 4835 4842 4848 4855 3 8 4861 4867 4874 4880 4887 4893 4899 4906 4912 4918 3-9 4925 4931 4937 4914 4950 4956 4962 4968 4975 4981 4 0 4987 4994 5000 5006 5012 5018 5025 5031 5037 5043 4-1 5049 5055 5062 5068 5074 5080 5086 5092 5098 5104 4-2 5111 5117 5123 5129 5135 5141 5147 5153 5159 5165 4.3 5171 5177 5183 5189 5195 5201 5207 5213 5219 5225 4-4 5231 5237 5243 5249 5255 5260 5266 5272 5278 5284 4 5 5290 5296 5302 5308 5313 5319 5325 5331 5337 5343 4-6 5348 5354 5360 5366 5372 5377 5383 5389 5395 5400 4'7 5406 5412 5418 5423 5429 5435 5441 5446 5452 5458 4 8 5463 54*9 5475 5180 5486 5492 5497 5503 5509 5514 4 9 5520 5526 5531 5537 5542 5548 5554 5559 5565 5570 5-0 5576 5582 5587 5593 5598 5604 5609 5615 5620 5626 5’1 5632 5637 5643 5648 5654 5659 5665 5670 5676 5681 5-2 5686 5692 5697 5703 5708 5714 5719 5725 5730 5735 5'3 5741 5746 5752 5757 5763 5768 5773 5779 5784 5789 5'4 5795 5800 5806 5811 5816 5822 5827 5832 5838 5843 5-5 5848 5854 5859 5864 5869 5875 5880 5885 5891 5896 (To be continued.)