462 - 2 - 



by a Oallneyer 'Reverse Telephoto' lens whicM has a n effective focal length of I inch. The 

 compar.it ively large distance of 2 inches from the rear component to the focal plane milies it 

 possiole to use this lens in spite of the space occupied Oy the rotatini glass block which is 

 a feature of the camera. 



NO time marker is ivaiUblo with the camera as manufactured. A crater tube (Sylvania 

 Type R.1130.S) has therefore been in3t-;lled to operate -it a tuning forl< controlled frequency of 

 5 kc/s. Thr light is focusscd, by mo'.ns of a small mirror and lens, on to the film on the 

 sprocket wheel in the space between the perforations and the edge of the film. The arrangement 

 Is shown in Figure 2. 



Light source. 



Light is provided by firing a sequence of flash bulbs, the wire-filled type being used in 

 preference to those with foil filling. The advantages are that, with the wirc-fillod variety, the 

 time interval between the application of voltage and maximum light output is constant within close 

 limits; the percentage of bulbs which fail to fire is very small; the bulbs do not fire 

 sympathetically and may thus be mounted compactly in groups without the use of partitions. They 

 are, moreover, robust, and able to resist explosive shock, 



Reflector units have been built to take twelve or twenty-four bulbs which are fired at 

 Intervals of 16 milliseconds. Their light outputs overlap to give an even illumination throughout 

 the sequence. 



Provision is made for firing a maximum number of 192 bulbs in groups over periods varying 

 from 80 to 320 milliseconds. The following formula, derived experimentally, gives the number of 

 flash bulbs required (bulbs used being Philips P.P. 56 with an output of 56,000 lumen seconds):- 



„ . f2 aLsu!^^ 



30 Z 



where f is the lens aperature. 



d is the distance from the light source to the subject in feet. 



is the water path in feet trav^.-rscd by the light between light source and camera, 



S is the speed of the camera in thousands of frames/second (with the Fastex camera the 

 exposure time is approximately one third the time per frame). 



Z is a factor which varies invcrs:ly with the spc^d of the film used and has a value of 

 unity for Kodachromc. 



N is the nuTibcr of bulbs in each group, the number of groups being determined by the 

 length of record required. 



Photographic layout . 



The layout is shown in Figure 1. The camera is mounted with its axis running vertically 

 through the centre of i inch plate glass window in the bottom of a brass box, (the waterscope). 

 The function of the waterscope is to eliminate optical distortion by the water surface, A mirror 

 is arranged beneath the waterscope at an angle of about 45° to the vertical, the depth and angle 

 of the mirror both being adjustable. A surface silvered mirror is introduced between the camera 

 and waterscope when photographs above the water surface arc required. 



Photography is carried ojt by reflected light against a black background. The reflector 

 units are clipped on to vertical poles which may be withdrawn from the weter in order to change 

 the bulbs. No waterproofing is provided as the bulbs operate quite satisfactorily under water. 



The explosive charge, a 15 grain electric detonator, is supported upon stiff wires which 

 serve also as firing lead. Orange and green coloured cable is used so that the colour rendering 

 may be checked in colour films. 



Pressure recording . 



Shock wave pressures are recorded for a period of 60 milliseconds by piezo-electric 

 technique. Tw i inch diameter tourmalint crystal gauges are connected to amplifiers and the 

 pressures recorded by an oscilloscope and drum camera. 



Time 



