xxxvi REPORT — 18G.J. 



In order to realize this generous bequest of Hofrath Schwabe, Mr. Loewy of 

 the Kew Observatory went to Dessau, taking with him a selection of dupli- 

 cate negatives and prints of the sun, which he presented, in the name of the 

 Association, to that gentleman. After receiving Mr. Loewy most courte- 

 ously, Hofrath Schwabe expressed his gratification at the high degree of 

 perfection attained in photoheliography, which surpassed his most sanguine 

 expectations ; he also handed over to Mr. Loewy's trust not only his valu- 

 able collection of sun-drawings, but also all his astronomical observations. 

 Some of these will be exhibited at the Association. 



It has long been a desideratum in photoheliography, with the view of ob- 

 taining the apparent diameter of the sun's disk, to ascertain the absolute values 

 in arc of the divisions of the measuring-instrument (Mr. De la Hue's Micro- 

 meter), and preliminary experiments were made with that object during the 

 period that the Heliograph Avas at the Cranford Observatory. These were only 

 partially successful. The mode of operation was this : a suitable object suffici- 

 ently distant was photographed by means of the Kew instrument, with the 

 lenses in the same positions as when solar pictures were taken. Different por- 

 tions of the object (windows, doors, &c. of a house, for example) were then care- 

 fully measured so as to ascertain their value in minutes and seconds of arc ; and 

 by measuring the pictures of these several portions with the arbitrary scale of 

 the Micrometer, the value of the latter in arc could be calculated. The experi- 

 ments did not succeed so well as could have been desired, in consequence of 

 the disturbance of the images by the undulations of the atmosphere, none 

 but very low objects coming within the desired range. More recently, how- 

 ever, the experiments have been taken up again with great promise, and 

 excellent photographs of the Kew Pagoda have been obtained, which possess 

 the requisite sharpness. The object itself, on account of its numerous galleries, 

 is peculiarly fitted for such' observations, as it will be possible to ascer- 

 tain and allow for any optical distortion of the photographic image. Thus 

 it is not improbable that the Pagoda will afford the means of ascertaining, 

 photographically, with the greatest accuracy, the angular diameter of the 

 sun, and will give data for correcting the assumed semidiameter of the moon, 

 by the discussion of photographic pictures of solar eclipses. 



M. Oussew has informed Mr. De la Eue that the Wilna Heliograph is 

 now at work under his direction, during the absence, on account of ill health, 

 of the Director, Prof. Sabler. At present he experiences some difficulty in 

 obtaining perfect photographs, and he has been invited to receive instruction 

 at the Kew Observatory. It is considered to be desirable on other accounts 

 that M. Gussew shoiild be able to avail himself of the advantage thus 

 offered to him, as an arrangement might then be made for the division of the 

 labour of reducing the Heliographic observations. 



An apparatus will shortly be added to the Kew Observatory for the important 

 object of the ready verification of sextants ; the system of distant mirrors 

 now in use, designed by Mr. Galton and erected at his own cost, being only 

 available in steady sunshine. 



The new apparatus has been designed by Mr. Thomas Cooke, the well- 

 known optician. In principle, it consists of four collimators fixed radially, 

 at various angles apart, round the table on which the sextant is to be laid 

 for examination. The cross wires of the collimators in each of their com- 

 binations are to be brought successively into contact by the sextant. Then 

 a comparison of its readings with the constant angles of construction of the 

 apparatus determines the error of the sextant at various points of its arc. 

 However, in practice, to avoid the cost of very large collimators, whose 



