579 
NATURE 
[AprIL 28, 1923 


I should be glad if any readers of NATURE would 
supply an explanation of the production of this 
image. Eric ROBINSON. 
Bedford School, Bedford, 
March 24. 

I must thank the Editor for the opportunity he has 
given me of examining the very interesting silvered 
reflector specimen submitted by Mr. Robinson. 
The image of the child’s head has evidently a 
photographic origin, as Mr. Robinson suggests, but 
that the glass was at some earlier time a photographic 
plate that had later been heated and allowed to settle 
down in a suitable mould seems to be improbable, for 
the following reasons : 
The image occupies only a small part, about an 
inch square, near the edge of the plate, the remainder 
of which shows no photographic details. There is, as 
Mr. Robinson remarks, no evidence of distortion at 
the curved portions, and a close examination of the 
surface shows none of those minute fractures that are 
usually visible when an old photographic plate is 
stripped and silvered. 
When the image is examined closely, it is seen that 
there is a sharp line of demarcation especially at the 
right-hand side and the bottom, which suggests that 
at some time the head has been cut from a photograph 
and pasted inside the clock face. As the result of 
contact, or possibly under the action of the light, the 
image has then been impressed upon the glass. At 
some later date the photograph has been removed but 
the image on the glass has persisted and been rendered 
visible by silvering. 
There are many examples of images being formed 
in the manner described on glass surfaces. A ‘“‘ To 
Let ’’ notice pasted inside a window often becomes im- 
printed on the glass, and the image may persist there- 
after for a very long time. Recently I observed on 
a tramcar window the wording of an advertisement 
that had been pasted on the glass and later removed. 
Silvering would of course make the images much more 
conspicuous. 
In the hope of being able to reproduce Mr. 
Robinson’s specimen, two photographs were attached 
with water inside a similar clock face and exposed to 
the light of an arc lamp for four hours. After the 
photographs had been removed the surface was 
thoroughly cleaned and silvered. Notwithstanding 
the briefness of the exposure, the images were then 
quite recognisable. 
As there was reason to think that the appearance 
might be wholly or partly attributable to contact 
rather than exposure to light, a glass surface was 
cleaned with caustic potash and upon it there was 
roughly sketched a face by means of a quill moistened 
with stannous chloride. The liquid was allowed to 
remain for two minutes on the surface of the glass, 
which was then recleaned by means of a cotton-wool 
pad and weak caustic potash solution. After silvering, 
the details of the sketch could be observed, although 
previously no traces were apparent. 
Another plate was similarly treated but not silvered. 
When this plate is breathed upon, the face can be 
distinctly seen. 
The subject is one that deserves fuller investigation. 
So materialistic an explanation as the above is not, 
I fear, so attractive as a psychic one. 
James W. FRENCH. 
Anniesland, Glasgow, April 9. 

Tactile Vision of Insects and Arachnida. 
Ir would be interesting to know more details of 
the research carried out by Mr. J. P. O’Hea on the 
““so-called eyes in insects and arachnida’’ (NATURE, 
NO. 2791, VOL. IIT] 

April 14, p. 498, in connexion with Commander 
Hilton Young’s suggestion), from which he arrives at 
the surprising conclusion that “‘ the organs generally 
known as eyes do not act as organs of vision.”’ The 
species Mr. O’Hea mentions are the house-fly, red 
ants, Tegenaria domestica, and “‘ many of the Epeire.” 
We have here an assortment of which the power, and 
even manner, of vision are scarcely comparable. 
Taking first the spiders, sight plays practically no 
part in the life of the common Epeirides ; the eyes, so 
far as one can see, simply serve to distinguish light 
from darkness, and form no clear image. This is 
not quite true of Tegenaria (the other spider men- 
tioned). A sudden movement of the hand, when the 
spider comes out to take a fly, I have known to send 
it back (this is also true of Agelena labyrinthica and 
others). In this case, however, it is a large moving 
object which frightens the spider, and it will hesitate 
in its attack if the insect cease to struggle, so that 
it does not find it by sight. 
As for the red ants (the species is not stated), we 
have the mass of Lord Avebury’s work, as well as 
that of Forel and many others, in determining the part 
vision plays. One of the simplest cases is that quoted 
by Forel (‘‘ Senses of Insects,’’ pp. 124-128), in which 
he found that specimens of Formica pratensis experi- 
enced considerable difficulty in finding the nest when 
their eyes were varnished (the antennary sense, how- 
ever, playing the most important part). 
It is in the case of the house-fly that Mr. O’Hea’s 
conchisions are most surprising. He maintains that 
if one jgradually brings the hand up to a fly on a 
window-pane, ‘‘if it be a vigorous specimen, [it] will 
evade the caress,’’ whereas if one approaches it from 
the other side of the pane the fly takes little notice. 
His conclusion is that the fly recognises the approach 
of the hand, not by vision but by currents of air due 
to the motion of the hand or by convection currents 
due to heat of the same. 
I have lately had occasion to catch a number of 
flies (Musca domestica and Calliphora vomitoria), and 
have found that one of the best ways was to bring 
a glass tube slowly and continuously up to the fly 
(any sudden movement almost always causes the fly 
to escape). If the movement is quite steady, the 
fly does not realise the situation until covered by the 
tube; it cannot apparently appreciate a slow move- 
ment. : 
When the fly is on the other side of the glass we 
have several factors to consider. For example, if the 
fly is outside, its field of vision below itself will be 
limited owing to bright reflections all round (its eye 
being close to the glass) ; hence movements from inside, — 
even if the fly could see below itself, would have to 
be sudden and on a larger scale to disturb it. 
The most obvious test to apply is as follows: 
Approach the under side of the fly (1) through glass, 
when, as stated, it often takes no notice ; (2) through 
trellis (such as a meat-safe is made of), when, in my 
experience, the same thing occurs. This seems to 
dispose of the idea that the fly is affected by con- 
vection currents. The explanation of the facts I 
should suggest to be somewhat as follows: The sur- 
face of the compound eye available to the fly for 
looking downwards is smaller than that on the top 
of the head. Moreover, the lower portion is never 
used when the fly is resting normally on a solid 
opaque body and the fly has only to take into account © 
An attack from below (when the 
attacks from above. 
fly is at rest) is outside its normal experience. One 
may recall also that the ocelli are situated on the top 
of the head and are usually considered to be useful 
for close vision. 
fly varnished. 
. 
The experiment, to be in any way 
conclusive, should be repeated with the eyes of the 
; 
