[Illustration:  FIG 2.—­THE RULE, WITH THE DOMINOES (1/4 Actual Size.) ]

Figures are characters without physiognomy, if we may so express ourselves, while the spots on the dominoes take particular arrangements according to the number represented, and differentiate themselves more clearly from each other than figures do.  They are at the same time more easily read than figures or regularly spaced dots.  Now, it is very important to fix the attention upon the numbers, since they are arranged at distances expressed in dioptries and indicated by the number of the spots.  On looking through the aperture, we see in the first place one of the dominoes more distinctly than the rest.  Then, on endeavoring to see those that are nearer or farther off, we succeed in accommodating the eye and in seeing the numbers that express the extreme terms of the accommodation, and consequently the amplitude.

[Illustration:  FIG. 3.—­DETAILS OF EYE PIECE.]

Let us now take some examples:  If we wish to express in dioptries the myopia of a person, we put the apparatus in his hand, and ask him to place his eye very near the aperture and note the number of spots on the most distant domino that he sees distinctly.  This is the number sought.  If the observation be made through the upper lens, it will be necessary to subtract five from the number obtained; if, on the contrary, the other lens is used, it will be necessary to add six.

If it is a question of a presbyope, let him look with his spectacles, and note the nearest domino seen distinctly.  This will be the number of dioptries expressing the nearest point at which he can read.  This number permits us to know whether it is necessary to add or subtract dioptries in order to allow him to read nearer by or farther off.  If, for example, he sees the deuce and the ace distinctly, say 3 dioptries or 0.33 meter, and we want to allow him to read at 0.25 meter, corresponding to four dioptries, it will be necessary to increase the power of his spectacles by one dioptrie.

Upon the whole, Dr. Bull’s optometer permits of measuring the amplitude of accommodation, and, consequently, of obtaining the approximate age of people, of knowing the extreme distances of the accommodation, and of quickly finding the number of the glass necessary for each one.  It reveals the defects in the accommodation, and serves for the quick determination of refraction.  So, in saying that this little instrument is very ingenious and very practical, Dr. Javal has used no exaggeration.—­La Nature.

* * * * *

THE SANITATION OF TOWNS.[1]

  [Footnote 1:  Abstract from the presidential address delivered
  before the Association of Municipal and Sanitary Engineers and
  Surveyors, at the annual meeting in Leicester, July 18, 1887.]

By Mr. J. GORDON, C.E.