There are no bounds to the malignancy of the impure spirits. They are sly and notable liars--they can assume the form of mortals who have passed over and they can assume personality and knowledge that was known to the dead. By degrees they inculcate evil, predisposing the victim to accept and yield to evil in particular forms. Frequently they proceed by slow stages, advising and inspiring savage asceticism, but seizing each stage of natural reaction from this unnatural régime to further subvert their victim in wantonness. The obvious need is for some method of distinguishing between good and bad projections of astral light. To the human eye alone there is no means of distinguishing between the etherealizations of the psychoplasm of the believer and the identical luminous phenomena which occur when there is a materialization of the actual spirit. It is there that psychic science can come to our assistance. The fluorescent bodies zinc sulphide, barium platino-cyanide, and the preparation known as Sidot’s hexagonal blonde, are all intensely susceptible to radioactivity. The rays of radioactive bodies have the peculiar property of being able to penetrate the ether, and the mass of spirit teaching tells us that this property is also common to the disembodied spirits of those who have passed to other planes. The relative purity or potency of astral lights may be readily ascertained by their effect upon a simple instrument that I have named the Psycho-Lastrometer. This instrument is both cheap and easy to make in the simple form in which I first used it. The later applications which make it a registering instrument in addition to being a mere indicator are necessarily costly, but these are only necessary to the expert investigator and are of no value to the mere seeker after proof or those who seek communion with the spirits of the dead for the purposes of solace, quasi-religious conviction, or vulgar curiosity. To make a crude psycho-lastrometer all that is necessary is a wide-mouthed glass jar whose walls should not be more than two millimetres thick. The height of the jar should be some eight inches, the width in proportion three and a quarter inches. I have found that an ordinary lipped beaker of Bohemian glass such as is readily obtainable from any maker of laboratory apparatus is admirably suited to the purpose. The neck of this jar must be fitted with a large cork or wooden bung the whole of which is covered with tinfoil. The centre of this cork should be pierced by a piece of brass wire five inches long, bent at one end to form a hook. This end is inside the jar and from the hook hangs the plate of the lastrometer. To the projecting end of the brass wire outside the jar should be soldered a circular collecting disc of brightly polished brass or tinplate three inches in diameter. This should stand up vertically to the axis of the wire, being thus on edge instead of forming a flat table.