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Updated: May 9, 2025
I may add that if this spherometer is placed on a plate of glass and exact contact obtained, and then removed, and the hand held over the plate without touching it, the difference in the temperature of the glass and that of the hand would be sufficient to distort the surface enough to be readily recognized by the spherometer when replaced.
We come now to an instrument that does not depend upon optical means for detecting errors of surface, namely, the spherometer, which as the name would indicate means sphere measure, but it is about as well adapted for plane as it is for spherical work, and Prof.
Herein, however, lies the great difficulty in using this instrument, i.e., to know the exact instant of contact of the point of screw, a, on the surface, f. Many devices have been added to the spherometer to make it as sensitive as possible, such as the contact level, the electric contact, and the compound lever contact. I am indebted for this plan to Dr. Alfred Mayer.
Harkness has been, using one for some time past in determining the errors of the plane mirrors used in the transit of Venus photographic instruments. At the meeting of the American Association of Science in Philadelphia, there was quite a discussion as to the relative merits of the spherometer test and another form which I shall presently mention, Prof.
To measure a plane or curved surface with this instrument, a perfect plane or perfect spherical surface of known radius must be used to determine the zero point of the division. Taking for granted that we have this standard plate, the spherometer is placed upon it, and the readings of the divided head and indicator, d, noted when the point of the screw, a, just touches the surface, f.
Harkness claiming that he could, by the use of the spherometer, detect errors bordering closely on one five-hundred-thousandth of an inch. Some physicists express doubt on this, but Prof. Harkness has no doubt worked with very sensitive instruments, and over very small areas at one time.
I have already quoted you the words of Prof. Harkness. Dr. Hastings, whose practical as well as theoretical knowledge is of the most critical character, tells me that he considers it quite easy to measure to 1/80000 of an inch with the ordinary form of instrument. Here is a very fine spherometer that Dr. Hastings works with from time to time, and which he calls his standard spherometer.
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