[To THE EDTPOR Or TIM " SPECTATOR:] Sin,—May I be
allowed to add an illustration to those which Mr. George Moores alludes to in his letter in your last issue on this subject of the superiority in certain departments of thought and work of the British to the metric units P The ease to which I desire to draw attention is that of optics and optical appliances. This is a department in which certain units are unalterably fixed for us by natural conditions, be our conventional standards what they may. One of these, and the most important of all, is the "working distance "—or most practically useful focal length—of the normal human eye. I say "most useful focal length" because the eye, as is well known, can be accommodated, like a pair of opera- glasses, for various focal distances. That at which we see best, which gives us the greatest available magnifying power by the use of the unaided eye, is ten English inches. In the British unit of an inch it is expressible by the unitary symbol 10. People who make a fetish of the metric system call this fundamental unit "250 millimetres." Again, our high-power microscopes have been gradually standardised, more or less, to a certain pattern, and a certain micrometer scale has come into very common use for microscopical measurements, the graduations of which have various values, according to the varying magnifying powers of the instru- ments with which the micrometer is used. With the standard type of high-power instrument the lignes of this scale can be read as one-ten-thousandth part each of an English inch. Again, the principal unit of measurement is expressed by a unitary symbol in British units. Expressed as a fraction of a metre, it must be read as 2a or 21-is according to the adjust- ment of the instrument, the symbo`l a signifying the metric unit, which is equal to one-thousandth part of a millimetre. The Continental makers instruct you to read the unit as 2a. Yet once again. The finest images that we can produce will 'bear enlargement to such a degree that we can just discern as measurable in the object a magnitude equal to one-tenth of the last-mentioned scale, or just about one-hundred-thousandth part of an inch. Objects smaller than this can be seen, but not measured with any certainty, because of the want of definition, which softens the edges of even the best images. Here, again, a fundamental natural unit is expressible by a unitary symbol in terms of the British inch. In terms of the metric system it must be written 45.—I am, Sir, &c.,
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