In view of the observations made of late years on the action of red light in variola, it was thought that it might be of interest to ascertain, if possible, whether the red, or other rays of daylight, exerted any influence in cases of vaccinia; that is to say, with a disease the specific virus of which differs from that of variola probably only in that its pathogenic capabilities are modified, while vaccinia presents clinically many points of resemblance to variola.

1. There were 90 patients at the Eastern Hospital, who, during the years 1897, 1898, 1899, 1904, 1905 and 1906, were injected with horseserum for a second attack or relapse of diphtheria, who had been treated with horse-serum at the first attack.

2. Of these 9 had an “immediate reaction,” 22 an “accelerated reaction,” and 8 had both, 39 in all, or 43·4 per cent.

3. The “immediate reaction” showed itself a few minutes to six hours after the first injection for the second attack.

4. The shortest interval between the first injection at the primary attack and the occurrence of an “immediate reaction” after a reinjection for a second attack was 35 days; the longest was 363 days.

5. The ”accelerated reaction” showed itself 19 hours to five days after the first injection for the second attack.

6. The shortest interval between the first injection at the primary attack and the occurrence of an “accelerated reaction” after a reinjection for a second attack was 25 days; the longest was 1511 days. In each of these cases the reaction occurred the day after the reinjection.

7. In seven of the cases of “immediate reaction” the symptoms were severe. In all save one of those of “accelerated reaction” they were mild or moderate.

8. In 24 of the cases there was a normal serum-reaction after the reinjection for the second attack; but even this occurred in most cases slightly earlier than is usual after injection at a primary attack.

9. So far as the figures go they show:

(a) that when a patient has had a serum-reaction at the primary attack, he is more likely to have an abnormal reaction after injection at the second attack;

(b) that the greater the volume of serum given in the primary attack, the more likely is an abnormal reaction to occur after serum at the second attack;

(c) and that consequently an abnormal reaction is most likely to occur after serum at a second attack when the patient has had a large volume of serum followed by a reaction at the first attack.

There are, however, indications that the volume of serum given at the primary attack may have different effects as regards the determination of an “immediate” or an “accelerated reaction,” according to its amount.