204 



NATURE 



[June 26, 1890 



THE PULKOVA REFRACTOR. 



which, however, 

 results : — 



has been inferred, give the following 



ON [the completion of the Pulkova Observatory, the 

 jubilee of which has recently been celebrated, the late 

 W. Struve published his "Description de I'Observatoire," 

 which made the scientific world acquainted with the com- 

 plete equipment of that institution. The additions which 

 have been recently made to the Observatory, in order to 

 preserve its high character and deserved reputation, have 

 induced the authorities to publish what may be regarded 

 as a supplement to that work, and the details now given 

 of the history of the erection, and the results of a sys- 

 tematic examination, of the new refractor are not less 

 interesting than were those of the old 15-inch. 



The optical work of this recent addition, as is well 

 known, is the work of Messrs. Alvan Clark, and the 

 parallactic mounting that of Messrs. Repsold, and both 

 of these eminent firms appear to have given, in their re- 

 spective departments, complete satisfaction to the Russian 

 authorities. Considerable difficulty was experienced in 

 procuring the necessary disks for the object-glass, but 

 eventually M. Feil, of Paris, supplied both flint and 

 crown. The former appears to have given perfect satis- 

 faction, but in the latter, near to the centre of the disk, 

 there is collected, about a quarter of an inch below the 

 surface, a quantity of small air-bubbles, which cover a 

 space one and a half inch long by one-eighth broad. As 

 in the opinion of the opticians, as well as of Prof. Picker- 

 ing, this defect would not introduce any inconvenience, it 

 was determined to proceed with the manufacture, rather 

 than to wait for a more satisfactory casting. This de- 

 fective spot, of elliptical shape, has no bad efifect on the 

 images of stars in the general use of the telescope, but 

 bright objects, such as a Lyras, are accompanied by two 

 streams of false light, some minutes in length, in opposite 

 directions, which appear to be produced by this defect in 

 the crown lens. The position angle of these rays is 

 found to be 1 1 4°-294°, and this direction is almost ex- 

 actly perpendicular to the major axis of the air-bubble, 

 which has been measured 7.f-2of. Moreover, as this 

 peculiarity is the more noticeable when the diameter of 

 the object-glass is diminished by diaphragms, there can 

 be no doubt that it is the result of diffraction produced 

 by this spot. 



The mounting of the object-glass in its cell differs in 

 two respects from the plan generally adopted. The in- 

 ternal surfaces of the two lenses are separated by about 

 six inches. Though this separation does not render the 

 telescope available for photography, it doubtless tends to 

 improve the achromatism ; and, further, since openings 

 are left in the cell for the purpose of cleaning the inner 

 surfaces, currents of air can pass between the lenses and 

 promote an equality of temperature between them and 

 the atmosphere outside. The two lenses are not rigidly 

 mounted in their cell of cast-iron, but, to prevent any risk 

 of pinching or strain that might arise from the unequal 

 expansion of metal and glass, a space of o"5 mm is left. 

 It was conjectured that a displacement of the lenses, rela- 

 tively to each other, through this small amount would 

 exercise no bad effect on the quality of the images, and 

 this anticipation has been found correct. 



The constants of the object-glass are as follows : — 



Red, 

 Yellow, 

 Green, 

 Blue, 



\ = 636 

 \ = 589 



^ = 535 

 \ = 481 



Focal length is'Sga 



„ 13-885 



„ 13-884 



13-892 



from which it will be seen that the achromatism is satis- 

 factory for the brighter parts of the visible spectrum, and 

 in fact accords with that part of the spectrum which was 

 originally selected for the minimum focal length, viz. 

 X = 000057. 



The relative position at the focus for rays of different 

 refrangibility was more accurately determined by the 

 method of Prof. Vogel with the aid of a small spectro- 

 scope, as well as with the great spectroscope attached to 

 the instrument. It was then seen that the part of the 

 spectrum between D and d was so nearly linear that no 

 certain determination of the difference of lengths for the 

 different colours could be effected. For more distant 

 parts of the spectrum the following measures were made 

 of the distances of the three hydrogen lines from the 

 normal position D-d: — 



C c// 

 F 



30 



6-4 

 32-9 



^f- 



o '00045 

 0-00233 



It is not uninteresting to compare this result with that 

 which Prof. Vogel obtained from measurements on the 

 Vienna refractor of 26 inches, where the general character 

 of the achromatism is very similar to that of the Pulkova 

 refractor, since in both the red images are joined between 

 D and F, and beyond F a rapid increase in the secondary 

 spectrum is exhibited — a defect common to all objectives 

 of silica glass. 



In the Vienna object-glass the distances of the focus of 

 the three rays before mentioned from the focal plane 

 D-d are — 



mm. 



C 27 



60 

 23-5 



Consequently the diameters of the circle of chromatic 

 aberration, reckoned on the same plane, are, for the two 

 telescopes, as follows : — 



Pulkova. 

 mm. 

 Aperture 762 



Focal length 14, 120 

 Diameter C 0-162 or 2-37 



„ F 0345 „ 5-05 



„ Hy 1775 ,.25-95 



Vienna, 

 mm. 



Aperture 675 



Focal length 10,360 „ 



Diameter C 0-176 or 3-51 



F 0-391 „ 7-81 



Radii of the crown-glass lens 



Thickness of the crown 

 Thickness of the flint 



/ - 5-1054 



\ + 5-2831 (computed). 



mm. 

 42-42 

 26-06 



Radii of the flint . 



f + 4-8386 

 \ - 140-130 



The focal lengths computed from these data, one of 

 NO. 1078, VOL. 42] 



Hy I -831 ,,30-48 



The advantages of a proportionately greater focal 

 length in the case of the Pulkova instrument are shown by 

 the somewhat smaller values of the angular diameter. 

 This want of perfect achromatism makes itself felt in the 

 Pulkova instrument in the images of stars remote from 

 the optical axis. For a circle about 16' in diameter, no 

 appreciable effect is noticeable, but outside this radius the 

 image has a tendency to exhibit a red fringe on the 

 side turned towards the optical axis, and a violet on the 

 side more remote. 



The parallactic mounting appears to possess and retam 

 a very satisfactory stability. In the case, however, of 

 exceptionally heaw object-glasses, it is of interest to 

 rigidly investigate' the flexure of the tube. The total 

 weight of the object-glass and cell is in this case 400 lbs. 

 approximately, and considering the great distance from 

 the centre of the instrument at which it is supported, the 

 I coefficient of flexure might be expected to be large. As a 



