478 



KNOWLEDGE & SCIENTIFIC NEWS. 



[July, 1906. "^ 



simple enough in itself, but, as applied to photographic 

 exposures, it may mean three distinct things, and which 

 of the three it means I have never been informed when 

 buying a shutter. It may signify (i) the whole dura- 

 tion of the exposure; (2) the efi'ective duration of the 

 exposure; or (3) the equivalent exposure. With all 

 slnitter.s, the exposure begins and ends gradually, and 

 the "effective duration" neglects the time taken up at ihe 

 beginning and end of the exposure when the light 

 action is so small as to be negligible. The " equivalent 

 exposure " is the duration of an exposure that would give 

 the same light-action if the opening and closing re- 

 quired no time at all. If the photographer has a satis- 

 factory amount of exposure with an aperture of //16, 

 and one twenty-fifth of a second, and he wants to 

 shorten the duration of the exposure and still maintain 

 the same amount of light-action by increasing the 

 aperture of //8 and the shutter speed to one-hundredth 

 of a second, then he wants equivalent exposures marked 

 on his shutter, and this, I believe, is what most people 

 do want. But if he calculates from the rate of a mov- 

 ing object and the permissible blur, the maximum 

 duration of exposure that will conform to his condition^, 

 then he wants to know the effective duration of the 

 exposures. I believe that such calculations are very 

 rarely made in practical work, even in those cases 

 where they would be simple and advantageous. The 

 whole duration of the exposure is of use only in a 

 secondary sense, or under very exceptional experi- 

 mental conditions. 



A A'cii/ SliutUr. — It is risky tO' call anything new. 

 But I believe the adjective is strictly applicable to a 

 shutter that will shortly be put on the market by Messrs. 

 Taylor, Taylor and Hobson. It embodies several new 

 details, the consideration of which will be found in a 

 communication made bv Mr. William Taylor to the 

 Royal Photographic Society, and published eighteen 

 months ago in their Journal. The shutter is diaphrag- 

 matic, and acts the part of a diaphragm as well as a 

 shutter by means of a simple contrivance that limits 

 the movement of the leaves, so that the maximum open- 

 ing is the aperture desired. .\ separate iris diaphragm 

 being unnecessary, the space that has to be left between 

 the lens components for the working of the shutter 

 is practically the same as is required for the usual 

 diaphragm alone. The shutter has four leaves, arranged 

 like an iris diaphragm, but the edge of each is so cut 

 that the opening made is always eight-sided, a much 

 nearer approach to a circle than is often obtained in 

 such apparatus. The opening and closing is done 

 always at the same rate, the varying periods of ex- 

 posure being obtained by stopping the movement for 

 the desired time when the opening is at its maximum, 

 and not by slowing the moving parts, so that the longer 

 the exposure the greater the efficiency. For the most 

 usual exposures, the twenty-fifth of a second and 

 longer, the efficiency is, I believe, from over 90 to about 

 97 per cent., and the difference between such figures and a 

 hundred is negligible in general work. The shorter ex- 

 posures are arranged to give the due proportion of light 

 effect; they are definitely made to be the equivalent ex- 

 posures, as explained above. The example I have gives 

 exposures from one second to the one-hundredth of a 

 second. The pneumatic break is exceedingly compact 

 and efficient, and all the mechanism is enclosed so that 

 it is protected as far as possible from dust. I have 

 referred rather in detail to this particular apparatus 

 because it seems to me a distinct advance in shutter 

 mechanism, and, indeed, to leave very little, if anything, 

 to be desired as a shutter for general use. 



ASTR.ONOMICAL. 



By Charles P. Bitlek, A.K.C.Sc. (Lond.), F.R.P.S. 



Constant Deviation Prisms for Radial 

 Velocity Determinations. 



.An interesting property of tlie constant deviation prism 

 was demonstrated recently by T. H. Blakesley, showing that 

 by employing two prisms of the same angle, but inverted 

 with respect to their bases, there would be produced two 

 spectra of opposite sense. When a star was brought into 

 the field the two spectra would, of course, have their colours 

 ia opposite directions, and the adjustment would be such 

 as to make any particular line coincident in the two spectra 

 if the light source were at rest; for a body in motion the 

 line would be split up into two moving in opposite direc- 

 tions in the field of view. Measurements of the distance 

 apart of these lines will give the necessary data for calcu- 

 lating the radial velocity of the star. 



Work at Greenwich Observatory. 

 May. 1905-May, 1906. 



In his report presented to the Board of \ isitors on May 30 

 the .\stronomer-Royal briefly summarises the various 

 classes of investigation which have occupied the staff during 

 the past year. 



Transit Instrument. — This has been repaired, and owing 

 to some difficulty connected with the repolishing, the instru- 

 ment was out of use for about two months. During the 

 rest of the year 6,335 transits were observed, and 5,531 ob- 

 servations with the circle. Two hundred and fifty-seven re- 

 flection observations of stars were made, and numerous 

 determinations of level and collimation error. The transits 

 are completely reduced to 1905, December 31, and apparent 

 K..\. is formed to igo6, May 6. The circle observations are 

 completely reduced to 1905, December 31, and as far as 

 apparent X.P.D. to 1906, May 6. The new working cata- 

 logue of stars of magnitude 9.0, and brighter, between the 

 limits of+-24<' 10+32° of N. Declination, forming reference 

 stars for the Oxford .Astrographic Zone, has been completed, 

 and includes more than 12,000 stars, the places of which 

 have been brought up to igio. A new determination of the 

 pivot errors was made in November, showing them to be 

 insensible. 



Altazimuth. — Various adjustments have been made of the 

 object-glass mounting, and the instrument mainly used as 

 a reversible transit. Observations of the sun, moon, planets, 

 and fundamental stars have been made throughout the year. 



28-Inch Befractor. — This has been used for micrometric 

 measures of double stars, diameters of Jupiter and its 

 satellites. 



Thompson Equatorial. — With the SO-inch refltctor photo- 

 graphs of Jupiter's satellites \T. and VII., minor planets, 

 Nova -Aquilae, Comet a (1905), and various nebula; were 

 obtained during the year. With the 26-inch refractor 72 

 photographs of Neptune and its satellite were taken on 28 

 nights, and are now being measured. 



Spectroscope. — A number of experimental spectra of sun- 

 spots have been photographed in the 3rd order spectrum. 



Astrographic Equatorial. — One hundred and si.xty-four 

 photographs have been taken on 60 nights. Various re- 

 measures of the catalogue plates have been made in the 

 zones 80° to the Pole. A table is given showing the num- 

 bers of separate stars measured for zones of 5" in the section 

 allotted to Greenwich, from which it appears that consider- 

 ably more than six times the number of stars will be in- 

 cluded in the resulting catalogue than are contained in the 

 Bonn Durchnuisterunr}. Reproductions of enlarged prints of 

 the chart plates have been made as far as possible. 



