
Dec. 15, 1870] 
NWALORE 
125 


of water, and receives in the normal condition the rays of light 
through air, I thought I might make my subaqueous lens of 
the same media. A couple of watch-glasses, placed with their 
concayities towards one another, so as to enclose a convex lenti- 
cular portion of air, when immersed in water, disperse the rays of 
light and diminish the size of objects seen through them, because 
they force the more refractive medium, the water, to assume a 
concave shape in relation to the air between the glasses. The 
same watch-glasses placed with their convex surfaces towards one- 
another, and connected round their edges by a water-tight 
rim, thus enclosing a concave lenticular portion of air, when 
Dr. Lankester and the Scarlet Fever Epidemic 
PERMIT me to make a few remarks on some notices of my 
paper on scarlet fever, published in your pages on the 17th of 
November last. Referring to my recommendation as to the 
destruction of the poison of scarlet fever, the Pall Mall Gazette 
says—‘‘ All this is very well in its way, and may be carried out 
by the upper and middle classes, among whom the mortality from 
scarlet fever is comparatively small ; but the plan is quite out of 
the reach of poor creatures who have but one room, one bed, and 
| one suit of clothing, which even at night takes the place of 
immersed in water, refract the rays of light convergently toa focus | 
and magnify objects, because they force the more refractive 
medium to assume a covvex shape in relation to the air between the 
glasses. Their magnifying power or focal distance under water 
is somewhat less than that of the same glasses in the reversed posi- 
tion and filled with water is in air; the slight difference being owing | 
to the greater refractive power of the glass in air than in water. 
1 found that two glasses of a curvature of about 14 inch radius 
thus placed formed in water a lens having a focus of about two 
inches. 
the loss of our anterior lens in water, and restores perfect vision. 
Of course the same magnifying power may be obtained by various 
combinations of differently curved glasses, or by plano-concave 
or concavo-convex air-lenses. The advantages of this kind of 
lens for subaqueous vision over a glass lens are obvious. It can 
be made of any required size so as to command a large lateral 
field of vision. It ceases to act as a lens the instant it emerges 
from the water, and does not interfere with vision in the air, as 
then we merely look through two thin pieces of glass with some 
air between them. There is no provoking loss of refractive 
power, as in the case of the glass lens; and lastly, it can be 
made very cheaply. With either form of lens we can see from 
below the water objects in the air above us quite distinctly if the 
surface of the water is smooth, less distinctly if it is agitated. 
Air lenses constructed on the principle described may be made 
of any magnifying power, and are much better adapted for the 
microscopic examination of objects under water than glass lenses, 
whose refracting and magnifying power is reduced to one-fourth 
by immersion. Thus a glass lens of a quarter-inch focus in air, 
would scarcely be equal in water to an air lens of one inch focus. 
I have said that the difference between the refractive power of a 
glass lens in air and water is as 4 to 1, or even more. The differ- 
ence is about the same in the case of the crystalline. Thus, the 
spherical lens of a cod, which has a focus of about 3 of an 
inch in air, has a focus of about = of an inch in water, which | 
is about the distance of its posterior surface from the retina in 
the fish. Supposing the focal distance of the human crystalline 
to be, in air, 4 of an inch, it will be more than } of an 
inch in the fluid in which it floats. But, in front of it, we 
find what I have called the anterior lens—I mean the aqueous 
humour—with a focus, as I have proved, of about 2 inches. 
If we take two lenses respectively of 2 inches and 3 of an 
inch focus, and place the weaker over the stronger, we shall 
find their united focal distance to be about $an inch, or about the 
distance between the back of the human crystalline and the retina. 
My measurements, in the absence of appropriate instruments, lay 
no claim to exactness ; they are, however, a sufficient approxima- 
tion to truth for my present purpose. 
How is it that after the operation of extraction of the crystal- 
line lens, which has a focus of less than 1 inch in its natural 
position, the patient can see distinctly with a lens of from 3 
to 4 inches focus? The reason seems to be that the optical 
character of the eye is completely altered by the operation. The 
space formerly occupied by the crystalline is now filled with 
aqueous and vitreous humour, and the eye represents a sphere of 
water, bulging in front into a more convex form by means of the 
cornea, which will have the effect of a superimposed meniscus of 
about 2 or 24 inches focus. A thin glass sphere filled with water 
of 1 inch diameter will roughly represent the eye deprived of 
its crystalline. We find the focus of this sphere to be about 4 an 
inch. Let us place in front of it a lens of 2% inches focus to 
represent the bulging cornea, and we find the focal distance 
diminished by more than one-half. Another lens of 3 to 4 
inches focus will bring this focus close to the posterior sur- 
face of the sphere, in fact, to the situation of the retina in the 
actual eye. This explains what happens in the eye deprived of 
its crystalline. Such an eye will require a much more powerful 
lens for subaqueous vision than that above described. 
Montagu Square R, E. DupGEon, M.D. 
This aév-/ens, as it may be called, completely suyplies | 

| Government that would reach even the 
blankets.” My object in writing the paper was to show that 
scarlet fever might be averted by certain measures, and I left it 
to those who read it to devise the means of making them avail- 
able for all. When the cattle disease broke out, an Act of 
Parliament was passed for the purpose of diverting it. The 
lives of human beings are surely of not less value even in a 
money point of view than those of cows and oxen, and I have 
the conviction that certain measures might be adopted by the 
**poor creatures” to 
whom the Pad? Mall Gazette alludes. Even now there exist Acts 
of Parliament which, if at once put in force by boards of 
guardians, town councils, vestries, and other local authorities, 
would at once enable them to put down this disease. The 
inhabitants in ‘‘one room, one bed, and one suit of clothing,” are 
reached by medical men, and they might be empowered to 
remove the sick from the healthy, to destroy useless infected 
clothing, to have the infected linen washed, and generally to 
see that the disease is arrested. What can be done amongst the 
rich ought to be done amongst the poor, and expense ought not 
to be allowed to stand in the way of such merciful measures. It 
should be remembered that such outlay on the part of wealthy 
ratepayers would, in the end, repay them, as they catch this 
disease from its being fomented among the poor, and they would 
no longer be liable to these attacks when their less opulent 
neighbours were free from them. 
In the pages of the Zamcet ‘‘ A General Practitioner”’ states 
that I have reflected on the members of the medical profession 
in stating that they do not exert themselves to suppress this 
disease. I spoke from a rather extensive experience on this sub- 
ject, and regret to say that I have nothing to withdraw on this 
point. I did not say it was the fault of medical men, 1 said 
they were not instructed. This is the fault of a system of medical 
education in which public health is not contemplated as a part of 
its course. It is true that within tie last two or three years 
chairs of Hygiene have been established at University and King’s 
Colleges, London, but these are exceptional. ‘So deficient is 
the education of medical men on this very point, that the 
Government felt itself justified in opening a special medical 
school at Netley for the purpose of supplementing the defects of 
our ordinary medical schools. It is from the Chair of Hygiene at 
the Military Hospital that the most admirable work on Hygiene 
in our language, by Dr. Parks, has issued. 
“« A General Practitioner” could not suppose that I was ignorant 
of the fact that all that has been done for our knowledge of the 
nature of contagious diseases had been done by medical men, 
and that our medical officers of health have especially exerted 
themselves in endeavouring to prevent the spread of contagion. 
I must, however, again express my surprise at the small amount 
of information that can be gained from the text-books on the 
practice of medicine as to how best to prevent the spread of 
contagious diseases. 
I will not in your pages do more than allude to the offensive 
tone and expressions of ‘‘A General Practitioner,” but I may add 
that no amount of ‘‘support” I may have had or may expect to 
derive from members of my profession, will ever induce me to 
refrain from speaking the truth of them in the interests of the 
public. Iam, however, fully convinced that it is only by sucha 
course that I can hope to retain the respect and continued 
“support” of the more intelligent and honourable membcrs of 
my profession, EpWIN LANKESTER 
Professor Tait on Bain’s Logic 
IN your last week’s number, Prof. Tait publishes a portion of 
his Introductory Lecture to his class, in which he criticises cer- 
tain passages in my work on Logic, having reference to the 
doctrine of the Conservation of Force. Although I do not, in 
every instance, admit the justice of the strong condemnatory 
phrases used in the criticism, Iam aware of having committed 
