100 YEARS AGO

Mr. Marconi's Results in Day and Night Wireless Telegraphy. I can assure Prof. Joly that his explanation will not do. The observed effect, which if confirmed is very interesting, seems to me to be due to the conductivity, and consequent partial opacity, of air, under the influence of ultra-violet solar radiation. No doubt electrons must be given off from matter (dust as well as other matter) in the solar beams; and the presence of these will convert the atmosphere into a feeble conductor. Conducting power in the sea-water surface assists and guides the waves, retaining them in two dimensions after the same fashion as a telegraph wire retains them in one; but conductivity in the dielectric itself will tend to dissipate and enfeeble the waves, by a process of reflection resulting in some amount of distortion.

O. Lodge

From Nature 3 July 1902.

50 YEARS AGO

During the past three years, experiments have been conducted with subjects who have been given the task of solving mental-test items of a particular type. Suppose PC be the probability that a particular subject will continue to work at a particular problem for some time t before giving it up, and let PE be the probability that if a solution is recorded within this time it will be the correct one. If the universe of discourse is now restricted to correctly solved items, the probability (PS) that a particular correct solution will be returned within a period of t sec. after the moment the problem is presented will be a function both of the dynamics of the problem-solving process as such and also of PC.... Let PS be defined as the probability which would obtain if PC = 1 for t = ∞. No comprehensive statement can be made about a person's ability to solve a problem which does not involve at least these three probabilities, that is, any attempt to measure 'intelligence' by mental-test methods should involve assessments of PC, PE and PS.... D is the difficulty of the problem being considered, defined in the conventional but arbitrary fashion D = [100 − R], and R is the percentage of an adult British population (unselected) who achieve success with the item.

From Nature 5 July 1952.