On the use of the automatic photometer in photographic photometry
Baker, Edwin Arthur
MetadataShow full item record
The displacement of the eye of the astronomer by the photographic plate from the telescope and spectro-meter and its transfer to the measuring machine was so rapid that, about twenty years ago practical astronomy seemed likely to become a branch of applied photography. More recently the introduction of the photoelectric cell and improvements in the thermopile and selenium cell have extended this process of displacement, not from the telescope only, but also from the micro- scope of the measuring machine.. The sensational shift of the lines in the companion of Sirius, for instance, was measured by a thermo- electric device.i One of the first. to advocate the newer methods of measuring a photographic plate was the late E.' O.' Pickering`; his appeal for an automatic means of measuring was answered shortly after by the introduction first of the Koch and later of other instruments.' Fortunately or unfortunately, the manufacture of the photographic plate has become so specialised that, advances in technique must be left in commercial hands, it remains for the astronomer to see that the fullest use is made of its capabilities, and while the element of personal judgement in the measurement of the negative remains this duty cannot be said to have been performed.It is a natural tendency for the user of an elec- trical mesuring device to study the properties of the * Harvard Circular, 155, 1910. photographic plate rather than to devote much attention to the improvement of the measuring device, for: the rea- son that the accuracy is limited by the defects of the plate and the methods of development, the errors of measurement being negligible. The automatic instrument, moreover, throws into prominence the weaker parts of the images which passed unnoticed by the eye. It is a peculiar: feature of the eye, and one that leads to considerable error in some instances, that it is incap- able of perceiving gradual changes in illumination. An abrupt change of illumination of one per cent can be seen, a gradual change of one hundred per cent may pass unnot- iced; hence as a criterion of uniformity of illumination the eye is not merely useless, it is misleading. The use of low densities is an obvious advantage in astro- nomy, for they require less exposure than dense images, and an extension to fainter stars becomes possible. The idea of measuring the positions and magnitudes of star: images which are invisible on the negative may appear strange, it is nevertheless true that. a star image of mean density less than 0.03 will not be picked up by eye, whereas a suitable photometer will locate and measure images down to a mean density of 0.01. The behaviour: of the photographic plate in this very low density region is of interest, not only for its astronomical applications, but also because of the simplicity of the results and the light they throw on the photographic action.The thesis consists of three parts. The first, which comprises chapters I to V, treats of the errors to be expected in automatic measures of stellar images arising from the properties of the photographic plate and from its treatment. The second, comprising chapters VI to VIII, is an account, of an attempt to measure the photo -visual magnitudes of the brighter stars of the North Polar Sequence as an example or the use of the photometer for image measurement, together with the considerations which led to the adoption of the method: used. The third comprises four appendices, containing that part of the work which is not of exclusively astronomical interest. The first of these is to be regarded as part of the main thesis - constitutes, in fact, the basis of the whole work.This thesis could not have been prepared without Professor Sampson's consent; but, the writer: is indebted to him for: much more than this would imply, and where the apparatus used is of an improvised or: inadequate type this is in no way due to his lack of sympathy with the work - on the.contrary, the writer has to thank him for the consideration he has given to requests, whether for apparatus or: for advice on points of difficult;.. The writer has also to thank Sir James Walker for facilities for: preparing photo electric cells in the Chemical Department, of the University of Edinburgh..