April, 1906 



KNOWLEDGE & SCIENTIFIC NEWS. 



397 



Guiding Telescopes. 



In carrying on photographic work with lenses thus 

 mounted the supporting telescope can always be used 

 for guiding purposes, the best form of eye-piece being 

 one of low power, fitted with cross wires sufficiently 

 coarse to be readily seen on an out-of-focus image of a 

 star. If the guiding telescope is of fairly large aper- 

 ture, it will be found in practice that a star sufficiently 

 bright can usually be found not very far from the centre 

 of the field covered by the plate in the camera. The 

 out-of-focus image of a faint star does not admit of 

 being enlarged to the same extent as that of a bright 

 one, and the cross wires should always be distinctly 

 seen, otherwise guiding, particularly if the driving 

 clock is not to be relied on, becomes a very tedious 

 operation. The eye-piece should be turned until one 

 of the cross wires exactly coincides with the path of a 

 star across the field when the telescope is stationary, 

 so that, apart from occasional slight alterations in De- 

 clination to correct for \arying refraction, the move- 

 ment of the telescope in Right Ascension will only have 

 to be attended to, and any irregularity in the driving 

 can be instantly corrected. 



Focussing. 



No difficulty need be experienced in adjusting the 

 camera to the correct focus of the lens, if the sun be 

 utilised for this purpose, and, when the position of the 

 screen is such that the solar image is most sharply 

 defined, this may for all practical purposes be accepted 

 as the correct focus for all celestial objects. When the 

 camera has been properly adjusted, and when this has 

 been verified by a few exposures, the two portions of 

 the box may be screwed together, as too much care 

 cannot be exercised with the view of eliminating every 

 possible source of movement which would be likely to 

 disturb the register of the plate. If a screen of ground 

 glass is used for focussing purposes it should be of the 

 finest grade procurable. Perhaps a more suitable 

 material is an ordinary gelatine plate, which has been 

 exposed for a few seconds to a diffused light, and after- 

 wards developed, fixed, and washed in the usual 

 manner. Some have recommended a piece of plain 

 glass with lines ruled upon it with a diamond. 



Defining Properties of Lenses. 



When the rirsl t.'\|j(isuri- lias been mack' a feeling of 

 disappointment may be exjierienced when it is seen that 

 tiif star discs are only sharply defined over a small 

 central portion of the plate, while towards the edges 

 they have the appearance of short circular trails. It 

 must, however, be kept in view that the area of critical 

 definiti(Mi with even the best portrait lenses, when used 

 without diaphragms, is usually smaller than the working 

 aperture, and if a 5 in. lens covers a quarter plate 

 sharply up to the edges, it may be regarded as having 

 excellent defining qualities. With the view of getting 

 good definition over a larger portion of the plate some 

 have suggested a method known as " averaging the 

 focus," i.e., adjusting the focus to an object some 

 distance from the centre of the screen. A few experi- 

 ments carried out in this direction will, however, clearlv 

 show th.il the method is not one to be recommended, as 

 it will tlKMi be found that ihe fainter stars at the centre 

 of the plate, instead of being minute points, are blurred 

 and enlarged. This applies with even greater force to 

 photographs taken at the primary focus of a mirror, as 

 the least alteration of the distance .separating the 

 speculum .and plate appreciably alters the definition of- 

 the whole field. 



(To be continued. ) 



MoLgnetism acnd the 

 Corona. 



Dr. Trowbridge's Experiments. 



In the course of an investigation made by Dr. John 

 Trowbridge, and reported by him in the Americafi, 

 Journal of Science, on the phenomena presented by 

 electric discharges in strong magnetic fields, he found 

 that with high voltages and strong, steady currents, 

 phenomena began to appear which were absent at 

 lower voltages and lesser currents. For the purposes 

 of the experiments, the description of which we are 

 about to recapitulate, Dr. Trowbridge employed com- 

 paratively large tubes; and while the voltages ranged 

 from 3,000 to 8,000 between the discharge ter- 

 minals, the currents ranged from five milliamperes to 

 twenty milliamperes. The cylindrical glass tubes con- 

 taining rarefied air, which were used in the following ex- 

 periments, were 30 centimetres long and four centi- 

 metres in diameter; and for resistances E)r. Trowbridge 

 made use of a column of running tap water. 



At pressures varying from i cm. to i m.m., the 

 cathode light on a circular aluminium plate, forming the 

 pole of a powerful magnet, the magnetic lines of which 

 were directed along the line of electric discharge, was 

 driven to the circumference of the disc, forming to the 

 eye an apparently steady circular discharge. When the 

 tube was covered, however, with black paper, so that 

 only the light on the disc could be seen, if this 

 light were examined in a revolving mirror, an interest- 

 ing case of unipolar rotation was seen. Fig. i is a 

 photograph taken of the reflection in the revolving 

 mirror. 



The glass walls of the tulje through which the photo- 

 graph was taken and the necessary obliquity in the re- 

 flection caused by the mirror, modify the sharpness of 

 the image. But the revolution of the image round the 

 pole is apparent. The speed of revolution was found to 

 increase with the degree of exhaustion of the rarefied 

 air in the glass tube. IVcsumably when the free path 

 of the ions increases, the progressive effect along the 

 magnetic lines becomes more than the rotational effect 

 of the magnetic field. When the plate formed the 

 anode and also the end of a magnetic pole, so that the 

 lines of magnetic force were directed along the line of 

 electric discharge, the light at the anode was separated 

 into two distinctly different lights, one (in rarefied air) 



