The Photographing of Patients

Common's photograph of the Orion nebula led, through Holden, to the acquisition by the Lick Observatory of a 3-foot reflector in 1895. Two mirrors made by Calver, formerly owned by Corn mon, were presented to that institution by Mr. Crossley of Halifax, England, and one of these, refigured by Grubb, forms the essential part of the instrument known by his name. In Keeler's hands it yielded a superb series of photographs of nebulae which for the first time demonstrated the great advantages of the reflecting telescope for certain classes of astronomical photography and led to the construction of the great modern reflectors.

Charts of the Heavens.

The advantage that photographs of clusters and nebulae have over visual observation is obvious. A glance at a photograph shows the whole of the object in all its beauty. The field of view of a telescope is small, and a complete conception of the nebula has to be built up from successive ele ments. It has already been mentioned that Gill foresaw the possi bility of recording the positions of stars from a single plate; the next step was to connect individual plates to make a complete survey of the heavens, as had been, and is, the final aim and pur pose of transit-observing. The first effort in this direction was made by Gill, who (1885-89), took photographs covering the sky from the South Pole to declination 18° S. which, measured by Kapteyn of Groningen, gave approximate co-ordinates and mag nitudes of nearly half a million stars that form the Cape Photo graphic Durchmusterung. In a more ambitious scheme initiated in 1887 by Admiral Mouchez, director of the Paris observatory, 18 observatories of the world combined to make the Astrographic Chart, or a picture of the whole sky, which should show all stars down to the i4th magnitude, to which was added later the making of a catalogue of all stars to magnitude i 1, now considered the more important part of the work. Space does not permit descrip tion of its details and methods. It must suffice to say that a great portion of the catalogue is already published and that, though some sections of the Chart are completed, the end is not yet in sight.

In this connection mention is to be made of a series of plates that came later, taken by Franklin Adams, a man well known in business circles in the City of London, in 1903, 1904, at the Cape of Good Hope and afterwards at Godalming, Surrey, with a photo visual objective (v. p. 837) of to in. diameter, devised for the work by the firm of Cooke and Sons. These plates each cover an area of 15°X 15°.

The extreme delicacy of the gelatine film, especially during de velopment and fixation, led in the beginning to a certain distrust of measurements made on photographic plates. Experience has shown

these fears to be groundless and that photographic methods are well suited to measurements of the most exacting kind. With properly designed instruments, the economic factors are also fa vourable. A recent undertaking, proposed by Schlesinger, which ef fectively utilizes these advantages, is the photographic reobserva tion of the zones of the great Astronomische Gesellschaft cata logue, originally measured with meridian circles at many different observatories. As an early example of astronomical work effected by the measurement of photographs mention may be made of the determination of stellar parallaxes by Pritchard at the Oxford University Observatory in the year 1887, and the remark made at that time that this initiated a new method of finding the dis tances of stars is amply justified, for to-day many observatories are pursuing that plan. Most researches that depend on the meas urement of small distances are now made by means of the photo graphic plate, close double star observing being an exception. Hertzsprung has devised methods involving the use of a grating and colour filters to ensure freedom from error in finding the dis tance between the components of double stars and their position angles, but it is considered that the photographic method is un suited for pairs whose separation is less than about a second of arc. Photographing stars through a grating of parallel wires placed before the objective, which produces a pair, or perhaps a series of pairs of diffraction images of each star, is a scheme prac tised in other branches of modern astronomy and astrophysics.

Besides the advantages already mentioned, others that pertain to the sensitive plate are that its receptivity is cumulative; by con tinued application it can see where the eye is blind. The impres sions are permanent and constitute evidence that can be referred to' at pleasure. Faint comets are discovered by photographing the re gion in which the object is believed to lie and searching the devel oped plate. A similar method for finding minor planets has been very fruitful and examination of the series of celestial photographs of the Harvard College Observatory has brought to light many Novae that have appeared in past years but have escaped detec tion. This collection of photographs which has been accumulated systematically over many years constitutes a record of the state of the sky at approximately regular intervals. It has been espe cially fruitful in the discovery of variable stars.