Spectra and Color
Brace’s Spectrophotometer Prism #10565
In D.B. Brace’s research work he made use of spectrophotometers of various kinds. Not satisfied with them, he invented a simple, but effective new type that utilized two 30°-60°-90° prisms cemented together with a silvered strip between them. His device was subsequently manufactured by Schmidt & Haensch in Berlin, Germany and Gaertner in Chicago and was used, e.g., at the Yerkes Observatory. Several of his prisms remain in the department.
References: D.B. Brace, "Description of a new spectrophotometer and an optical method of calibration," Philosophical Magazine 48, 420-30 (1899); D.B. Brace, "On a new system for spectral photometric work," Astrophys. J 11 (1900): 6-23. Schmidt & Haensch Katalog II: Spektralapparate, 1900 and Katalog II Spektralphotometer, Pyrometer, etc., 1914.
Large Prism Spectrometer #10060
Franz Schmidt & Haensch, Berlin, S.
The Schmidt & Haensch Company of Berlin offered this spectrometer for use with D.B. Brace’s spectrophotometer prisms. The main scale is calibrated in units of 10 minutes of arc and with the vernier it can be read to 10 seconds of arc. In addition to the usual telescope for viewing and the collimator for the light source, this instrument has a second collimator tube to utilize his new spectrophotometric system which Brace described in the literature. He attached the department’s 4-inch Brashear telescope, directed at the star Capella, to one collimator and a standard light source to the other. This enabled him to make quantitative measurements of the spectrum of the star.
References: Schmidt & Haensch Catalogue No. 55 (1896); D.B. Brace, Philosophical Magazine and Journal of Science, 48, 420 (1899); D.B. Brace, Astrophysical Journal, 11, 6-23 (1900).
Direct Vision Spectroscope #10076
Unsigned, probably German, late 19th century
A train of prisms of alternating types of glass is able to disperse a collimated beam of light into its spectrum without deviating its direction appreciably.
Reference: Max Kohl Catalogue No. 100 (c.1927) p.417.
Ultraviolet Spectrometer #10339
R. & J. Beck, London
Many of the important lines in atomic spectra lie outside the visible range, especially in the ultraviolet. Since ordinary glass does not transmit UV very well, special glass, quartz, or other crystalline minerals such as rock salt are used. A fluorescent screen allows one to view the otherwise invisible spectrum. This simple spectrometer has a scale reading from 2000 to 4500 Ångstrom units in steps of 100 Å.
Reference: Ralph Sawyer, Experimental Spectroscopy, New York, 1944.
Color Blindness Tester #10325
This is the Holmgren Color Test consisting of skeins of different colored yarn. The subject selects samples that match each of three standards. From the numbers selected the type of color blindness or lack of color blindness can be determined. The Physics Department acquired the kit in January 1922.
Rowland Gratings #10648, 10649, 10659, 10660, and 10661
Henry A. Rowland and John A. Brashear
In 1882 Henry Rowland (1848-1901) at Johns Hopkins University devised an extraordinarily uniform screw which he used as the heart of his new ruling engine. With this system he produced diffraction gratings that were by far the best made at the time. For many years John Brashear made the speculum metal concave mirrors on which Rowland and his helpers ruled gratings with tens of thousands of lines per inch. Because of the uniformity of the grating spacings they avoided the "ghost" lines seen with less uniform gratings. Rowland’s gratings, which were sold at cost to investigators all over the world, created a revolution in spectroscopy. The 6-inch grating, which was the largest size produced by Rowland, was probably purchased about the same time as a smaller one dated 1889. D.B. Brace also had a special grating made for his abortive attempt to measure the velocity of light in 1889.
References: Gerard L’E Turner, Nineteenth-Century Scientific Instruments, Berkeley, 1983, pp. 161-63; D.B. Brace, Science n.s.16, 81-94 (1902); John A. Brashear Company Catalogue Optical, Physical, Astrophysical, and Astronomical Instruments, 1906.
Echelon Grating #10333
Albert Michelson invented the echelon, which acts like a diffraction grating by virtue of a division of the wave front into parts that travel through different thicknesses of glass and are reunited at an angle depending on the wavelength. Echelons provide a very high resolution but since the orders overlap, they are generally used with a prism that allows only a narrow region of the spectrum to reach the echelon. Glass of very uniform thickness has to be used in the construction of echelons. Petitdidier advertised that his echelon plates differered from each other in optical thickness and parallelism by no more than one-twentieth of the wavelength of light. He made most of the echelons used in the United States.
References: George Monk, Light Principles and Experiments, New York, 1937, p.199-202; Deborah Jean Warner, "Octave Leon Petitdidier: Precision Optician," Rittenhouse 9, 54-58 (1995).