"These ultra-violet rays," he continued, "are always present in an electric arc light though not to a great degree unless the carbons have metal cores.They extend for two octaves above the violet of the spectrum and are too short to affect the eye as light, although they affect photographic plates.They are the friend of man when he uses them in moderation as Finsen did in the famous blue light treatment.But they tolerate no familiarity.To let them -=20particularly the shorter of the rays - enter the eye is to invite trouble.There is no warning sense of discomfort, but from six to eighteen hours after exposure to them the victim experiences violent pains in the eyes and headache.Sight may be seriously impaired, and it may take years to recover.Often prolonged exposure results in blindness, though a moderate exposure acts like a tonic.
The rays may be compared in this double effect to drugs, such as strychnine.Too much of them may be destructive even to life itself."Prescott had now paused and was regarding Kennedy contemptuously.
Kennedy paid no attention, but continued: "Perhaps these mysterious rays may shed some light on our minds, however.Now, for one thing, ultra-violet light passes readily through quartz, but is cut off by ordinary glass, especially if it is coated with chromium.Old Mr.
Haswell did not wear glasses.Therefore he was subject to the rays - the more so as he is a blond, and I think it has been demonstrated by investigators that blonds are more affected by them than are brunettes.
"You have, as a part of your machine, a peculiarly shaped quartz mercury vapour lamp, and the mercury vapour lamp of a design such as that I saw has been invented for the especial purpose of producing ultra-violet rays in large quantity.There are also in your machine induction coils for the purpose of making an impressive noise, and a small electric furnace to heat the salted gold.Idon't know what other ingenious fakes you have added.The visible bluish light from the tube is designed, I suppose, to hoodwink the credulous, but the dangerous thing about it is the invisible ray that accompanies that light.Mr.Haswell sat under those invisible rays, Prescott, never knowing how deadly they might be to him, an old man.
"You knew that they would not take effect for hours, and hence you ventured the prediction that he would be stricken at about midnight.
Even if it was partial or temporary, still you would be safe in your prophecy.You succeeded better than you hoped in that part of your scheme.You had already prepared the way by means of a letter sent to Mr.Haswell through Dr.Burnham.But Mr.Haswell's credulity and fear worked the wrong way.Instead of appealing to you he hated you.In his predicament he thought only of his banished daughter and turned instinctively to her for help.That made necessary a quick change of plans."Prescott, far from losing his nerve, turned on us bitterly."I knew you two were spies the moment I saw you," he shouted."It seemed as if in some way I knew you for what you were, as if I knew you had seen Mr.Haswell before you came to me.You, too, would have robbed an inventor as I am sure he would.But have a care, both of you.
You may be punished also by blindness for your duplicity.Who knows?"A shudder passed over me at the horrible thought contained in his mocking laugh.Were we doomed to blindness, too? I looked at the sightless man on the bed in alarm.
"I knew that you would know us," retorted Kennedy calmly."Therefore we came provided with spectacles of Euphos glass, precisely like those you wear.No, Prescott, we are safe, though perhaps we may have some burns like those red blotches on Mr.Haswell, light burns."Prescott had fallen back a step and Mrs.Martin was making an effort to appear stately and end the interview.
"No," continued Craig, suddenly wheeling, and startling us by the abruptness of his next exposure, "it is you and your wife here - Mrs.
