218 



KNOWLEDGE. 



[No^■EMBER 1, 1900. 



The Paris siderostat is shown on Fig. 2, which was 

 specially drawn for the readers of Knowledge. Its 

 total length is 27 feet and such is its height also. It 

 weighs, moreover, some 45 tons. The glass mirror itself, 

 whose diameter measures 78| inches, or more than 

 6i feet, and whose thickness is 11 inches, weighs 

 3| tons. It is held in equilibrium by a system of levers 

 and counterpoises, and rolls in a cylindrical well con- 

 taining 22 gallons of mercury. The volume of the sub- 

 merged part was so calculated that the weight borne by 

 the mercury is nine-tenths of the joint weight of the 

 mirror and its support, that is, nearly 13 tons. 



This description of the mechanism will be rendered 

 clearer by an inspection of Figs. 3 and 4, showing the 

 axes, circles, forks, and mirror of the siderostat, while 

 the Plate gives a general frontal view of the instrument. 



Fig. 3.- 



-Right Ascension and Declination Axes and Circles of the 

 Great Siderostat. 



The mirror was cast by M. Despret, in June, 1895, 

 at the glass works of Jeumont. The object glass was 

 cast by M. Mantois, while all the mechanical part of the 

 apparatus, including the figuring of the optical surfaces, 

 was made by M. P. Gautier, optician to the Paris Obser- 

 vatory, whose plans were carried out thoroughly bv 

 M. G. Allix, a workman of great skill. 



It was no easy task to polish the surfaces of the 

 colossal mirror and of the lenses of the object glass. For 

 this, M. Gautier had to devise a new method. The 

 grinding action of two flat metallic sliders gave to the 

 mirror its flat surface, while the same process was used 

 in figuring the object glass; owing, however, to the 

 slight curves to be given to the surfaces, the slides, 

 instead of being straight, presented the cui-vature of the 

 disks. The rectilinear motion of the system thus 

 developed gave rise to a cylindrical section, which, how- 



ever, in virtue of the rotation of the lenses round their 

 axes was transforaied into a spherical surface. 



In testing the mirror, M. Gautier followed Foucault's 

 process, which consists in examining telescopically the 



Fig. 4. — Xhe Great Mirror, 79 inches across. 

 {Photographed i'l/ M. G. Mathifu.) 



image of a point of light reflected from the mirror. If 

 the surface be quite plane the image reduces itself to a 

 small luminous circle surrounded by concentric dif- 

 fraction rings. If the portion of the surface under 

 scrutiny is slightly concave, there will be a flattening 

 of the image in the vertical direction, when pushing the 

 eye-piece in, and it will be elongated in the same 

 direction when drawing the eye-piece out. Should the 

 surface be slightly convex, the reverse would take place. 

 While making these experiments, M. Gautier noticed 

 that the mirror's sensibility was such that bv merely 



Fig. 5.— The Object Glass of 4'J-2 inches. 



{I'hvloiirtij'ht't I'll M. Cj[. Maihikc.) 



touching the surface with the hand he produced a pro- 

 tuberance deforming the telescopic image at that point, 

 and which, measured with the spherometer, attained 

 YTooo of an inch. The spherometer, meantime, enabled 

 the detection of irregularities in the plane surface, not 

 exceeding ^suWu of ^^ inch. 



