It may appear somewhat surprising that the many thousands of intergalactic luminous formations were not discovered earlier. The reasons for this are treefold. Large reflectors and refractors were mostly built with small focal rations such as F/5 (for instance the 60-inch and 100-inch telescopes on Mount Wilson). Instruments of this sort are little suited for the photography of faint extended surface images. Much progress was therefore made as soon as Schmidt telescopes of large apertures and large focal ratios (F/2) became available. The second reason for possible failure lies in the frequent neglect of the observers to make their telescopes really light tight. How serious the effects of such neglect can be was experienced by the present writer when he attempted to photograph the luminous filaments between IC 3481 and IC 3483. Although these filaments were clearly recorded by the large Schmidts they could not be photographed with the 100-inch reflector until all sources of stray light were eliminated. Surprisingly enough there were about six sources of this kind including light from the night sky hitting parts of the exposed plates directly or by reflection and scattering on various surfaces. Even dust on the mirrors and poor silver or aluminum coats may be fatal. A third phenomenon which must be watched is the night sky glow. This glow, which may considerably change in intensity from night to night, is seldom explored prior to exposing critical plates. Much can be gained by the use of night sky meters measuring the spectral intensity distribution of the general background. In general the sky glow is the least intense in the green between 5100Å and 5500Å, in which range the variations from day to day also seem to be the smallest. Unfortunately photographic emulsions are generally quite insensitive in the green. Also, the luminous formation to be photographed may have its maximum surface brightness in another colour range than green, so that up to the present time the problem still remains of how to record luminous intergalactic matter with the optimum conditions of night sky and of instrumentation.
In addition to photography one may attempt photoelectric recording. Along this line the author a few years ago suggested plans to Dr. W. A. BAUM and F. E. ROACH to determine both the surface brightness and the colours of luminous intergalactic formations between the members of small groups of nebulae and within large clusters. Surprisingly enough results have not been forthcoming as fast as anticipated. On a first attempt Dr. BAUM did not succeed to record the photographically very conspicuous filament between the two nebulae shown in Plate IV. With his newer and much more sensitive pickup (22) which is supposed to measure surface brightness as faint as the 25th photographic magnitude per square second of arc Dr. BAUM in March 1954 has obtained readings on the above mentioned filaments, but has not yet reduced the data. On the other hand, at the meeting of the Astronomical Society of the Pacific in Pasadena of June 1955 he reported observations on distant clusters confirming the writer's photographic results concerning the existence of extended luminous clouds within the large clusters of galaxies (15).
In the meantime great progress has been made with the method of composite analytical photography which is particularly suited for enhancing the contrasts of faint objects against the surroundings and which constitutes a unique means for distinguishing between different colours (21).