June 7, 1894] 



NA TURE 



139 



.rge angle, the rays are refracted into the receiving hat, and 

 produce an effect much larger than when the prism is re- 

 moved. 



An ordinary 9 inch glass lens is next placed near the source, 

 and by means of ihe light of a laper it is focussed between source 

 and receiver. The lens is seen to increase the efifect. 



Aiago Disk, Grating and Zone-plate. 



The lens helps us to set correctly an iS-inch circular copper 

 disk in position for showing the bright diffraction spot. 

 Removing the disk, the effect is much the same as when it was 

 present. Add the lens, and the effect is greater. With a 

 difTraclion grating of copper strips two inches broad and two 

 inches apart, I have not yet succeeded in getting gool results. 

 It is difficult to get sharp nodes and interference efifects with 

 these sensitive detectors in a room. I expect to do better when 

 I can try out-of-doors, away from so many reflecting surfaces ; 

 indoors it is like trying delicate optical experiments in a small 

 whitewashed chamber well supplied with looking-glasses ; nor 

 have I ever succeeded in getting clear concentration with 

 thii zone-plate having Newton rings fixed to it in tinfoil. 

 But really there is nothing of much interest now in diffraction 

 effects except the demonstration of the waves and the measure 

 of their length. There was immense interest in Hertz's time, 

 because then the wave character of the radiation had to be 

 proved ; but every possible kind of wave must give inteiference 

 and diffraction etTects, and their theory is, so to say, ivorked 

 out. More interest attaches to polarisation, douole refraction, 

 and dispersion experiments. 



Polarising and Analysing Grids. 



Polarisation experiments are easy enough. Radiation from a 

 sphere is already strongly polarised, and the tulie acts as a 

 partial analyser, responding much more vigorously when its length 

 is parallel to the bne of sparks than when they are crossed ; but 

 a convenient extra polariser is a grid of wires something like what 

 was used by Hertz, only on a much smaller scale ; say an iS- 

 inch octagonal frame of copper strip with a harp of parallel 

 copper wires. The spark-line of the radiator being set at 45'', 

 a vertical grid placed over receiver reduces the deflection to 

 aht)ut one-half, and a crossed grid over the source reduces it to 

 nearly nothing. 



Rotating either grid a little rapidly increases the effect, which 

 becomes a maximum when they are parallel. The interposition 

 of a third grid, with its wires at 45° between two crossed grids, 

 restores some ot the obliterated effect. 



Radiation reflected from a grid is strongly polarised, in a 

 plane normal of course to that of the radiation which gets 

 through it. They are thus analogous in their effect to Nicols, 

 or 10 a pile of plates. 



1 he electric vibrations which get through these grids are at 

 nuht angles to the wires. Vibrations parallel to the wires are 

 rcll;;cied or absorbed. 



Ktflecting Prism. 



To demonstrate that the so-called plane of polarisation of the 

 transmitted radiation is at right angle to the electric viljration,' 

 i.e. that the wires of the grid are parallel to it, I use the 

 same paraffin pri,m as befjre, but this lime I use its largest 

 lace as a retleclor, and set it at something near the polarising 

 angle. When the line of wires is parallel to the plane of 

 incidence, in which case the electric vibrations are perpen- 

 dicular to the plane of incidence, plenty of radiation is 

 reflected by the paraffin face. Turning the grid so that the 

 electric vibrations are in the plane of incidence, we find that 

 the paraffin surface set at the proper angle is able to reflect 

 hardiy anything. In other words, the vibrations contemplated 

 by Kresnel are the electric vibrations ; those dealt with by 

 McCullagh are the magnetic ones. 



'I hus are some of the surmises of genius verified and made 

 obvious to the wayfaring man. 



A^ 



rilE REPORT OF THE ASTRONOMER ROYAL. 

 T the annual visitation of the Royal Observatory, Green- 

 wich, on Saturday last, the Astronomer Royal pre-enled 

 his report of the progress made from .May 11, 1S93, to May 10 

 of this year. We take (rom it the following information : — 



It appears that the average num ler of transits observed was 

 no less than 31 each day, or if Sjndays are excluded, 36. As 



1 c.f. Trjuton, in Natcrs, vol. 39, p. 353 ; and many other optical 

 experiments by Mr. Trjutjn, vol. 40, p. 39S. 



an instance of the number of observations which were made 

 under very favourable conditions, it may be mentioned that on 

 three consecutive days in February no fewer than 45S transits 

 and 460 zenith distances were observed. 



A new universal transit-circle or altazimuth is being con- 

 structed by Messrs. Troughton and Simms, and satisfactory 

 progress has been made towards completion. All the heavy 

 portions of the instrument, including the rotating and reversing 

 gear, are made, and have been put together, the object glasses 

 lor the instrument and collimators are practically finished, as 

 well as the eye end with its micrometers, and the circles, micro- 

 scopes, &c. , are in hand. 



As previously noted in our astronomical column, a valuable 

 gift has been made to the Observatory by Sir Henry Thompson, 

 who has generously offered a sum of /'5000 to (irovide a large 

 photographic telescope with accessories, which would serve as 

 the complement of the 2S inch visual telescope ju-t completed. 

 This munificent offer was readily accepted by the .-Vdmirally, 

 and after careful consideration and discussion, a photographic 

 telescope of 26 inches aperture and 22 feet 6 inches local length, 

 equatorially mounted, was ordered of Sir H. Grubb on May 5, 

 the instrument to be completed in eighteen months. This 

 telescope will be of exactly double the dimensions (aperture and 

 focal length) of the asrographic equatorial which has proved 

 so succesiful, and it will be mounted on a very firm stand which 

 will allow of complete circumpolar motion without the necessity 

 for reversal on the meridian, which has b;en felt as a drawback 

 in the astrographic equatorial. It will be erected on the 

 central tower of the new Physical Observatory, under the 30 

 leet dome which is shortly to be placed there, and will carry 

 the I2j-inch Merz refractor as a guiding telescope aid the 

 Thompson 9-inch pho'.oheliograph. It will thus be mounted 

 under very favourable conditions for work, and will be in every 

 respect a most effective instrument. 



The new 28 inch refr.tcio' has been brought into working 

 order after much time spent in the erection of the instrument, in 

 the adjustment of the object glass, and in the provision of 

 various fittings at the eye end. The adjustments were finished 

 by October i, when, under good atmospheric conditions, the 

 definition was found to be very fine- Since then the object 

 glass has been tested on various objects with very satisfactory 

 results. .A sketch of Jupiter and some measures of double stars 

 have been made, and the colour correction of the o'oject glass 

 has been determined on stars by readings for focus at different 

 parts of the spectrum. 



The object glass has also been tried in the photographic 

 position, with the crown lens reversed and the lenses separated 

 on the plan proposed by SirG. G. Stokes. The determination 

 of the best distance between the lenses and the exact adjustment 

 of the crown lens for tilt and centering relatively to the flint has 

 necessarily taken a long lime, as small modifications were re- 

 quired in the cells and special contrivances had to be devised 

 lor the delicate alj'jstment of the heavy crown cell and lens. 

 A large number of photographs have been laken at different 

 distances inside and outside of the focus coriesponding to dif- 

 lerent positions of the crown lens, and affording interesting 

 inforuiaion which will be useful in connection with the 

 Thompson 26inch photographic telescope. 



With ihe astrographic equatorial, 923 plates, with a total of 

 2143 exposures, were taken on 1S3 nights in the year ending .May 

 10. Of these iSl were rejected, owing to photographic defects, 

 mechanical injuiy, mistakes in setting, the plate being wrongly 

 placed in the carrier, failure in clock driving, and interference 

 by cloud. The following statement shows the progre-s made 

 with the photographic mapping of the heavens in the year 

 covered by the report : — 



No. of Successful 

 Photo.s taken. Plaies 

 Astrographic chart (exposure 40m.) ... 280 220 



I'lates for catalogue (exposures 6m., 



3'n. and 203.) ... ... .. 50S 3S7 



Number of fields photographed for the 



chart ... ... ... ... ... 200 



Number of fields photographed for the 



caLilogue ... ... ... ... 367 



Total number o( fields photographed 



since the commencement of the work 



for the chart ... ... 333 



Total nunifier of fields photog'aphed 



since the commencement of the work 



(or the catalogue ... ... ... 610 



NO. 1 284 VOL. sol 



