JufU 27, 1878] 



NATURE 



225 



adjusted that the image of the light source is seen in the 

 little telescope reflected by the general surface of the 

 metal grating. Supposing a circle and micrometer at- 

 tached, represented by the Avooden bar at the top, the 

 reading would now be zero. 



Next, and this is the new point, a piece of glass silvered 

 on the front surface is fixed with its surface parallel to 

 the surface of the grating, the side of which it covers. 

 When this is in perfect adjustment the images produced 

 by the movable grating and the fixed mirror are super- 

 posed. Let us suppose the light source to be a Geissler 

 tube, we get a single image of it ; the fixed mirror is then 

 very slightly inclined, so that its image lies a little above 

 or below the one due to the grating. We now by the 

 movable arm at top rotate the grating, the grating image 

 vanishes from the field of view, and in a little time, if the 

 rotation is continued, the blue of the first order spectrum 

 makes its appearance. Each coloured image in the spec- 

 trum can in turn be brought to coincidence, with the non- 

 dispersed image of the tube thrown by the fixed mirfor, 

 and readings of considerable accuracy can thus be 

 obtained. The illumination of the image due to the 



fixed mirror can be easily regulated by changing its 

 position with regard to the axis of the telescope pro- 

 longed ; in no case of course should any part of the ruled 

 surface of the grating be covered. With a ring slit illu- 

 minated by the vapours of different metals, the pheno- 

 mena observed are very interesting and novel ; with the 

 fixed mirror slightly inclined, the image from the fixed 

 mirror always in the centre of the field of view forms 

 a capital point of comparison. 



More light is gained by employing an object-glass of 

 short focus and placing the grating and fixed mirror at 

 such a distance inside the foeus that the beam falls on the 

 ruled surface and a small fraction of the fixed mirror. 



I hope the suggestion does not come too late to enable 

 it to be utilised by the outposts during the coming eclipse. 

 If it helps in enabling us to determine the position of the 

 chromospheric line near H, the time I have spent on the 

 little model will not have been thrown away. I may add 

 that I have found that a prism of 60° dense flint placed 

 in front of the lens of an ordinary photographic camera 

 will give us, if properly focussed, a most useful spectrum 

 of the eclipsed sun. J. Norman Lockyer 



OUR ASTRONOMICAL COLUMN 



Nearest Approximations of Small Planets to 

 THE Earth's Orbit.— Out of the 187 minor planets 

 now known there are ten which approach the earth's 

 orbit at their perihelia within 0*9 of her mean distance 

 from the sun, and which may therefore afford the mQSt 

 advantageous opportunities for determination of the 

 solar parallax by one or other method of observation of 

 these bodies, already successfully applied : Medusa is 

 omitted on account of uncertainty of elements. The 

 nearest approach, 0798, is made by Clio, discovered by 

 Luther in August, 1865. ^ihra, detected by Watson in 

 June, 1873, makes the nearest approach to the sun r6i4 ; 

 but the great depression of the planet below the plane of 

 the ecliptic, at perihelion, prevents so near an approxi- 

 mation to the earth's orbit as in the case of Clio. The 

 following is a tabular view of the distances in the ten 

 cases referred to : — 



Hellocento? . . Distance 



Latitude from 



in Perihelion. Earth's Orbit. 



+ i°57 ... 0-798 ,. 



- 23 45. - 0-813 



- 7 9 ... 0-815 

 + o 48 ... 0-818 

 + 7 40 ... 0^828 

 + 5 53 ••• 0-842 



- 5 45 .•• 0-874 



- o 53 ... 0-882 



- 6 53 ... 0-892 



- o 47 ... 0-896 



If we extended our limit to 1-0 we should include, in 

 addition to the above, Felicitas, Phocea, Euterpe, Thyra, 

 Echo, and Feronia. . 



While referring to the small planets it may be remarked 

 that, between the perihelion of jEthra and the aphelion 

 of Hilda, there is a difference of 2-98 ; and between the 

 aphelion of Flora and the perihelion of Hilda 076, or 

 upwards of three-fourths of the radius of the earth's 

 orbit. The periods of Flora and Hilda being respec- 

 tively 3-27 and 7-85 years exhibit a difference of 4-58 

 years. These are the extremes, as they result from the 

 latest and most complete catalogue of elements. 



According to the last Circular of the Berliner Jahrbuch, 

 the following names have been proposed : — For No. 177, 

 Irma, for 180, Garumna, and for 186, Celuta. 



Measures of Double Stars. — Many applications 

 for copies of the earlier volumes of " the Leyton Observa- 

 tions " having been received after the edition had beeh 

 exhausted, Mr. J. Gurney Barclay has issued a fourth 

 volume containing the double star epochs from the com- 

 mencement of observations at Leyton, with the addition 

 of results to the end of 1877. This part includes also 

 occultations and phenomena of Jupiter's satellites since 

 1865. The notes on the double-star observations coni- 

 prise the principal epochs of other observers. The smaB 

 companion of Procyon at a distance of about forty-five 

 seconds, to which attention was first pointedly directed 

 by Mr. Barclay in January, 1856, had the following posi- 

 tion for 1 863 23, angle, 294° '88, distance 45'''-9, which, 

 corrected for the proper motion of Procyon in the ihte'rval, 

 gives for 1879-0 angle, 3i9°"3, distance 47''-3. 



The Astrono7nische Nachrichten, Nos. 2196-99, contain 

 measures of double-stars made by Dr. Doberck at the 

 observatory of Col. Cooper, Markree Castle, Sligo, from 

 the end of 1875 to the spring of 1878. The list includes 

 most of the well-known binary systems, y Coronae was 

 single in the Markree instrument in 1876-77. ,. ' 



Mr. Ormond Stone, Director of the Observatory at 

 Cincinnati, writes with respect to a remark in a notice of 

 the Cincinnati measures of double-stars, which appeared 

 in this column, and which might be misunderstood as 

 implying that the work carried on at the American Obser- 

 vatory is to a certain extent a duplication of that com- 

 menced some time since with the refractor at Melbourne. 

 Mr. EUery, however, has lately informed Mr. Ormond 

 Stone that lais observations are limited to stars south of 35". 



The Binary Star a Centauri. — Mr. Maxwell Hall 

 writes from Jamaica, on May 21, with reference to a 

 Centauri : " Since my communication last year respecting 

 this binary, the angle of position of the smaller star has 

 rapidly increased at the rate of 60"" per annum. I have 

 lately taken measures in the same manner as before, few 

 in number, but with the greatest care, so that their con- 

 cordance gives them great weight." 



Epoch 1878-38 Position 139°- 1 Distance 2" '4 



Mr. Hall adds: "There can be no doubt that the 

 smaller star is variable : according to my estimates it has 

 diminished during the last year ; and I would therefore 

 call attention to the subject " — and appends various esti- 

 mates from \\ (Powell, Jacob) to 4 (Dunlop), also a table 

 of the measured angles and distances to 1878, which it is 



