290 REPORTS ON THE STATE OF SCIENCE, ETC. 



decrease the tendency to flicker at low speeds. Lecture rooms of from 120 to 150 

 seats will then be catered for, but this capacity is the limit that can be expected 

 from 16 mm. outfits. 



For lecture rooms of from 200 to 250 seats, the professional sized film of 35 mm. 

 is advisable to-day. In existing schools and colleges where structural alterations are 

 not intended or are difficult, safety base 35 mm. film combined with a projection outfit 

 which complies with the special Home Office regulations for portable projectors gives 

 satisfactory results. These regulations are the outcome of a committee of the film 

 trade working in collaboration with the Home Office officials, instituted to provide 

 for exhibitions in schools and parish halls, etc. It was considered by the trade that if 

 some such regulations existed, education committees and societies might provide 

 a good market. Several makers of apparatus produced quite low-priced outfits of 

 good quality, but even so, the outfit was found to be too costly for the parish hall, 

 anrl education authorities apparently still have the matter under consideration. 



For larger audiences, specially constructed theatres should be provided, with a 

 properly constructed operating room equipped with two projectors, the whole outfit 

 compl^-ing with the regulations made by the Home Secretary under the Cinematograph 

 Act and the particular local authority. The seating should be inclined and a good 

 lighting system should be provided throughout the theatre. 



The old difficulty of lack of electric current is quickl}'^ disappearing, and the 

 standardisation of current generally under the Central Electricity Board will also be 

 of great advantage. Where electric current is not available, the 16 mm. outfit may 

 be used with a 12 volt bulb running from batteries, the batteries to be of the 

 motor car type which can be charged at the local garage. The use of oxygen gas 

 from cylinders, and coal gas for lime lighting is quite safe when certain precautions 

 are taken. 



The incandescent focus lamp is being very rapidly improved on account of the 

 demand for it in the production of talking pictures. The 30 volt 900 watt lamp 

 has been found very satisfactory for demonstrations up to 200 people on the 36 mm. 

 film, and this type of lighting can be recommended for use wherever possible. It is 

 simple, satisfactory and safer than any other light source. 



A specification for a 16 mm. projector will be found in the Appendix. Such a 

 projector can be purchased nowadays through the ordinary channels for about £20. 

 A projector outfit to take standard (35 mm.) safety base films and complying with the 

 Home Office regulations for portable projectors will cost about four times that amount. 

 For a stationary projector to take standard films and suitable for professional use the 

 cost will be from £135 upwards. 



C. Illumination and Eye-stbain. 



Choice of Screen. — The size of screen adopted naturally varies according to local 

 conditions. A screen 5 to 6 ft. in width is usually ample for school use, but folding 

 screens of somewhat smaller dimensions are supplied with some well-known types 

 of portable projectors. When the film is shown in a long and narrow room, 

 aluminium-painted screens may be used with advantage. The brightness of the 

 image will then vary with the angle of view. The ratio of the brightness on the axis 

 to that obtained when the film is viewed from the extreme angle of observation should 

 not exceed 4 to 1. The limiting angle corresponding with this ratio may vary from 

 25° to about 40° according to the nature and size of the screen. In all cases where 

 observation at a somewhat oblique angle is likely, dead-white screens are preferable. 

 In this case the brightness on the axis is less than with an aluminium-painted screen, 

 but the effect, as viewed from different angles, is more uniform. 



Nature of Illuminant and Intensity. — In practice the illuminant is usually a focu;; 

 type electric incandescent lamp, which may consume 200-250 watts, and is run off 

 50 volts. When alternating current is available, the pressure may be conveniently 

 reduced by a transformer : otherwise a suitable resistance in series with the lamp is 

 necessary. Lamps capable of being run direct from the supply voltage are sometimes 

 used, but the effect is less satisfactory. With such a lamp a mean beam intensity 

 up to 2,800 candles may be attained, corresponding with a screen illumination of 

 about seven foot-candles at a distance of 20 feet (with no film in the gate and the 

 shutter open). 



Conditions of Display. — Apparatus of the above type may be used in rooms from 

 which daylight cannot be entirely excluded, but naturally no direct daylight should 



