might be of high economic value, where large numbers are involved, and might contribute much to the individual comfort of the workers.  But a constant relation to day and year also seems to exist independent of all personal variations.  When the sun stands at its meridian, a minimum of efficiency is to be expected and a similar minimum is to be found at the height of summer.  Correspondingly we have an increase of the total psychical efficiency in winter-time.  During the spring-time the behavior seems, as far as the investigations go, to be different in the intellectual and in the psychomotor activities.  It is claimed that the efficiency of the intellectual functions decreases as the winter recedes, but that the efficiency of psychomotor impulses increases.[44]

The influences of the daily temperature, of the weather and of the seasons may be classed among the physical conditions of efficiency.  We may group with them the effects of nourishment, of stimulants, of sleep, and so on.  As far as the relations between these external factors and purely bodily muscle work are involved, the interests of the psychologists are not engaged.  But it is evident that every one of these relations also has its psychological aspect, and that a really scientific psychotechnical treatment of these problems can become possible only through the agency of psychological experiments.  We have excellent experimental investigations concerning the influence of the loss of sleep on intellectual labor and on simple psychomotor activities.  But it would be rather arbitrary to deduce from the results of those researches anything as to the effect of reduction of sleep on special economic occupations.  Yet such knowledge would be of high importance.  We have in the literature concerned with accidents in transportation numerous popular discussions about the destructive influence of loss of sleep on the attention of the locomotive engineer or of the helmsman or of the chauffeur, but an analysis of the particular psychophysical processes does not as yet exist and can be expected only from systematic experiments.  Nor has the influence of hunger on psychotechnical activities been studied in a satisfactory way.

A number of psychological investigations have been devoted to the study of the influence of alcohol on various psychical functions and in this field at least the strictly economic problem of industrial labor has sometimes been touched.  We have the much quoted and much misinterpreted experiments [45] which were carried on in Germany with typesetters.  The workmen received definite quantities of heavy wine at a particular point in the work and the number of letters which they were able to set during the following quarter-hours were measured and compared with their normal achievement in fifteen minutes.  The reduction of efficiency amounted on the average to 15 per cent of the output.  It may be mentioned that the loss referred only to the quantity of the work and not at all to the quality.