On the Prissm. Spertrograph of the Touer Telesrope the Tokyo Astronomial Obseredtory in

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1 On the Prissm. Spertrograph of the Touer Telesrope the Tokyo Astronomial Obseredtory in (Fig. 1) This spectrograph eonsists of three orthogonal prisms of the same dimension, of which two are combined tosether, as we see in Fig. 2 and 3. The sides of the prism are 138,220 and 260 min. in length. The slit, the centre of the collimator lense and the centre of the prisms are loeated at the same level. The plate-holder is placed at a disetanice of 10con. below to the centre of the slit. In order to bring the spectrum on the central line of the photograhie plate, one side of the lense- Fig. 1. holder and of prisms must be inclined to a slight degree. The inclination is adjusted by moving the three bases of the main Idatu. Every part of the speetrum, from ultra-violet to infra-reel, is photographed by rotating Prism III which moves about the nxis *. In order to adjust this rotatios, a scale in useed, which can lit, read through telescope nose the plate-holder (Fig. 3). a The temperature in the basoment is kept censtant in order to avoid the variation of constants of the optical parts. 1. Experiment. Extending from Septamber to November in M u, we photographed the spectra of Fraunhofer lines between about 3500 A and 8200A (Figs. 4 and 5), for the purpose of determing the focus of the collimator

2 328 Yoshio FUJITA. [Vol. 16 Fig. 2 Fig. 3. I, II, III : three prisms; I and II are combined. a. Collimator lense : f=345 cm. Aperture =12 cm. b. Scale index which shows the rotation of Prism III, c. Screw. d. Slit and plate-holder. e. Axis of rotation of Prism III.

3 1931] On the Prism Spectrograph of the Tower Telescope. 339 lense and obtaining the sharp image of the spectrum, from ultra-violet to infra-red. The width of the slit, the inclination of the plate-holder and of Prism III and the focus of the collimator lense are necessary factors for obtaining good spectrograms. The photographic plate employed are TSyo panchromatic plates for the region of ultra-violet vnd violet, Illford special rapid plates for the visible region and T,',yo panchromatic plates sensitized with Riken Illuminol II for the infrared region. The exposure time extends from 1 second to 8 minutes, according to need. Fig. 4. H (the right) and n (the left) lines of the solar spectrum. Exposure : 5 and 10 seconds. A (the upper) and B (the lower) bands of the solar spectrum. Exposure: 5 and S miuut as. Fig. 5. Intensity curve of 11 and k lines.

4 330 Yoshio FUJITA. [Vol. 16 Dispersion and resolving power. The index of refraction is generally expressed by Kettler-Helmholz's iormuln as follows: By this formula the dispersion and the theoretical resolving power of prisms can be oasily determined. The angular dispersion is expressed as follows: where D and represent tho angle of refraction, the mob ser of prisms and the angle of incident ray respectively. The angular dispersion multiplied by the focal-length of tho collimator lense means the dispersion. When all the surfaces of the prism are used, we can simply express the theoretienl resolving power in the minimum deviation as follows: where B is the greatest depth of the ray which passes through the prism. Table 1 shows the numerical values of the linear dispersion (d), the indble of refraction ( ) and then resolving power (r), from 8500 A to 8500 A. Fig.6 is the graphical representation of these values. Fig. 6.

5 1934 the On Rrism Spectrograph of the Tower Telescope. 331 Table 1. The value of the dispersion here obtained is 0.78mm/A near 8950 A, the deviation between the observational and the theoretical value being about 0.02mm:A Now, for the purpose of investigating the relation between the rotation angle of Prism III and the spectral region on the photographic plate, we photographed the. Fraunhofer lines on the same plate, changing the rotation angle, and measured the change of the position of the Name line, according to the increase or decrease of the angle. Fig. 7 shows thin relation. The writer wishes to express his hearty thanks to Professor Sotome, Director of the Observatory, for his kind advice and to Mr. G. Yoshida for his kind help. Pig 7. Tokyo Astronomical Observatonry. (Reseived July