422 



NATURE 



{Feb. 28, i: 



the hour-circle, is connected with the axis of the outer mirror 

 by gearing, so that the observer at the stationary eye-piece has 

 both the hour and declination circles immediately under his eye. 

 He can thus direct the instrument to any object without moving 

 from his chair, and his observations are made under the most 

 favourable conditions for his own comfort, similar to those under 

 which the microscope is used by the student of natural history. 

 The observing-room, which may be artificially warmed, is quite 

 separated from the object-glass, and other external parts of the 

 instrument. These latter are protected from the weather by a 

 suitable hut, which can be rolled away on rails before observ- 

 ing, so that the optical parts of the equatorial are in the open 

 air under the best conditions for establishing an equilibrium of 

 temperature. 



The importance of obtaining the favourable conditions for 

 observation secured by M. Lcewy's equatorial coude has long 

 been recognized, and various attempts have been made to enable 

 the observer to command any part of the sky without changing 

 his position. In 1858, Dr. Steinheil proposed ^ a new method of 

 mounting a reflector, so that the axis of the concave mirror 

 formed the polar axis, the rays from a star being reflected down 

 the axis to the concave mirror by a plane mirror, which could be 

 rotated about a declination-axis and a polar axis. The observer 

 looked down the polar axis through a hole in the plane mirror, 

 but with this arrangement he could not observe stars much north 

 of the equator unless the plane mirror were made very large, 

 and the range of the equatorial was thus very restricted. A 

 more extended range might be obtained by interchanging the 

 concave and plane mirrors, so that the observer would look up in 

 the direction of the pole ; but the concave mirror and its support 

 would block out the view of the region near the pole, and of 

 all the sky below the pole. Sir H. Grubb has applied the 

 same principle to the construction of a siderostatic refractor. 



As compared with Dr. Steinheil's form, the equatorial coude 

 possesses the great advantage of commanding every part of the 

 sky, the arm of the telescope below the elbow being made long 

 enough to project beyond the sides of the observing-room when 

 viewing objects near the meridian. 



The siderostat of Foucault, though useful for many purposes, 

 is open to the same objection as Dr. Steinheil's, of not permitting 

 of a view of every part of the sky ; and there is the further 

 difficulty that the apparent direction of the diurnal motion is 

 continually changing. In the equatorial coude this direction 

 changes with the declination, but M. Loewy has now arranged 

 that the micrometer is turned with the declination-circle, and is 

 thus always set to the zero of position-angle. 



The success obtained by M. Loewy in the construction of the 

 equatorial coude is due to the following circumstances : — 



(i) The absence of flexure in the mirrors, which are made 

 much thicker than usual. 



(2) Themore perfect achromatism secured by the greater focal 

 length which this form of mounting allows of. 



The first condition was established by careful experiment, 

 which showed that in order to avoid deformation by flexure the 

 thickness of a mirror should be between one-fifth and one-sixth 

 of the diameter, instead of one-ninth or one-tenth as had been 

 usual hitherto. 



As regards achromatism, M. Loewy urges that, in order to 

 be able to see better with a larger object-glass, the achromatism 

 must be made more perfect, and that, therefore, the ratio of 

 focal length to aperture must increase with the aperture in order 

 to diminish the effect of the secondary spectrum. 



Notwithstanding the two reflections, the definition obtained 

 with the equatorial coudi appears to be very good, the com- 

 ponents of CO Leonis, distant only o""5, having been separated 

 with the Paris instrument, which has an object-glass of 0*27 

 metre or about io| inches. With one of the new instruments of 

 0-31 metre, or \i\ inches aperture, M. Trepied, at Algiers, easily 

 divided •f- Andromedae. The loss of light by the two reflections 

 from silvered mirrors is computed by M. Loewy at only 12 per 

 cent., and it would seem that it is at any rate very small, as suc- 

 cessful observations of a minor planet of 13 '5 magnitude were 

 obtained with the Paris instrument as well as of very faint 

 nebulae and comets. The comet 1885 a' (Fabry) was discovered 

 with this instrument. 



One of the objects which M. Loewy had in view in planning 

 his equatorial coude was to obtain greater stability than is 

 attainable with ordinary equatorials, and to make the measure- 

 ment of large angular distances possible. The form of mounting 

 ' Astron. Nachrichten, No. 1138 ; Monthly Notices, vol. xix. p. 56. 



of the equatorial ccude seems peculiarly adapted to give great 

 stability, provided the fixity of the mirrors in their cells can be 

 secured, and this is a condition to which M. Loewy has given 

 special attention. Each mirror rests in its cell on thick felt or 

 flannel, and is held by three clips, which are just brought into 

 contact with it when in the horizontal position, as tested by the 

 disappearance of the least trace of light between the clip and 

 its reflected image. This adjustment being made for the hori- 

 zontal position, in which the weight of the mirror has its full 

 effect, perfect contact between the mirror and its clips will be 

 maintained in all positions. 



M. Loewy, in conjunction with M. P. Puiseux, has investigated 

 very completely the theory of the instrumental adjustments of 

 the equatorial coude, including the effect of flexure of the polar 

 axis and of the telescope arm, and has shown the relation of his 

 formulae to those for ordinary equatorials. He arrives at the 

 two following conditions of optical adjustment as sufficient for 

 astronomical purposes : — 



( 1 ) The axis of the telescope arm should be perpendicular to 

 the polar axis. 



(2) The interior mirror should reflect to the centre of the field 

 a ray entering the telescope along the axis of the arm, supposed 

 to be perpendicular to the polar axis. 



The discussion of the instrumental errors of the Paris instru- 

 ment, partly by astronomical observations, and partly by means 

 of a collimator attached to the mounting of the exterior mirroi-, 

 shows a very satisfactory accordance in the determinations on 

 different days, and in the result the instrumental errors were 

 found to be very small, the largest amounting only to 23". The 

 coefficients of flexure are, however, rather larger quantities, 

 being 91" and 53" for the polar axis and telescope arm re- 

 spectively, as found by means of the collimator. It may be 

 expected that in the new instruments the effects of flexure would 

 be very much less, as important improvements have been made 

 in their mechanical construction. 



It is not a little remarkable that the first instrument made on 

 this new principle should have given such excellent results, both 

 optically and mechanically ; and its success is evidence of the 

 thoroughness with which M. Loewy has worked out his idea, and 

 of the skill with which MM. Henry and M. Gauthier have re- 

 spectively carried out the optical and mechanical portions of the 

 instrument. 



I now pass on to M. Lcewy's new method of determining the 

 constant of aberration. It is hardly necessary to insist on the 

 importance of this constant, not only for obtaining the true 

 positions of the stars, but, in a higher degree, for the determina- 

 tion of the solar parallax by means of the velocity of light. It 

 must be admitted that the nine independent determinations of 

 the constant of aberration made at Pulkowa with three different 

 instruments show a satisfactory accordance, but in the opinion 

 of M. Nyren, who has published the latest researches on the 

 subject, none of these can be asserted to be free from systematic 

 error. M. Nyren's definitive value is 20" -492, exceeding by 

 o"*047 W. Struve's original value, which has hitherto been 

 generally used by astronomers. Under these circumstances, M. 

 Lcewy's method, which is based on differential measures with 

 an equatorial, constitutes a new departure of great value in 

 astronomy of precision, and its value is enhanced by the circum- 

 stance that it is also applicable to the determination of the 

 constant and law of refraction. 



The principle of M. Lcewy's method is the measurement of 

 the angular distance between two stars by means of a double 

 mirror, formed by silvering two faces of a large prism of glass 

 and placed in front of the object-glass of an equatorial. The 

 double mirror is capable of rotation about the axis of the tele- 

 scope, so that by reflection from the two silvered surfaces the 

 images of two stars in different parts of the sky may be brought 

 into the field side by side, and the distance between them 

 measured in the direction of the common plane of reflection. 

 In his memoir on the determination of refraction by the new 

 method, M. Loewy proves that the projection of the distance 

 between the two images on the trace of the common plane of 

 reflection is independent of the rotation of the equatorial, of any 

 movements of the double mirror, and of the displacement of the 

 images by the diurnal motion, when the observation is not made 

 rigorously in the plane of reflection. 



M. Lcewy's exposition of his method of determining the con- 

 stant of aberration is contained in a series of communications 

 made to the French Academic des Sciences and published in the 

 Comptes rendus, vols. civ. and cv. In giving an account rf this 



