Gr. H. F. Nuttall 
491 
and Cooper’s dip. Oils of pennyroyal, bay and geranium appear to 
disturb the insects. Olive oil containing 10 and 25 % oil of cloves had 
no effect, whereas the lice attempted to feed but failed to do so when the 
skin was anointed with pure oil of cloves, oil of bergamot and terebene. 
They became immobilized in 4 minutes in contact with clove oil and died, 
but after a few hours recovered from the effects of terebene and bergamot. 
(4) It is commonly stated for instance that naphthaline sachets 
must exert a repellant action because persons wearing them remain 
free from lice. There is no scientific proof of this, for lice may be absent 
owing to other causes acting singly or combined, all the credit being 
given to the “repellant” whereas the individual freedom from lice may 
be due (a) entirely to the insecticidal effect of the substance or (b) to 
more care being taken to keep the individuals free from vermin. Some 
authors, as already stated (vide (1)), deny that they have seen any 
benefit from “repellants.” 
Critical Considerations and further Experiments. 
The contradictions contained in the statements above cited evidently 
requiring an explanation, I determined to make some further experi¬ 
ments to discover if there are substances which actually repel lice. 
The only experiments hitherto recorded which appeared convincing to 
me are those of Bacot (hitherto unpublished, see p. 489), but in my 
opinion they are somewhat vitiated by the manner in which they were 
conducted, the repelling effect of the substance tested being considerably 
obscured by the counter repellant effect of light itpon the insects. 
Even in the experiments which gave the most positive results many 
insects shunned the light more than they did the repellant (carbolic 
acid, naphthaline); the tests therefore give an unfair measure of the 
potency of the repellants tested. 
The other experiments indicating that lice may be repelled by certain 
substances (see p. 488) are open to the criticism that the authors did 
not perhaps guard themselves against obvious sources of error: 
(1) In Teske’s experiment the lice might have scattered anyhow 
without any repellant being present as I have frequently seen them do. 
(2) In Sergent and Foley’s experiment the influence of light was 
perhaps not considered; there is no proof that chance distribution was 
excluded and that both pieces of cloth were identical in texture. 
(3) In Jeanneret-Minkine’s experiment the same criticism holds as 
for that of Teske with the added factor that the lice may also have 
been repelled by the heat from the stove. 
Parasitology x 32 
