750 HANDBOOK OF PHOTOGRAPHY 



of known wavelengths of the spectrum, e.g., the principal absorption lines of hydrogen, 

 an accurate determination of the color curve of a lens may be made. A graph of the 

 results gives at once the relationship between the focal setting and the wavelengths of 

 the light passing through the lens. 



Measurements of the Photographic Plate in Astronomy. — The principal uses of 

 astronomical photographs may be divided roughly into two categories — qualitative 

 and quantitative. In the first category, astronomical photographs may be used for 

 The discovery of new stars, the so-called "novae," comets, and asteroids. Here the 

 important element is the existence of plates taken at different dates or times. The 

 use of a "blink" comparator, whereby it is possible for the eye to scan rapidly the 

 objects first on one plate and then on the other, facilitates very quickly the discovery 

 of an object which exists at a certain date that was not visible at a different epoch. 

 Similarly variations in the brightness of stars known as "variable" stars may be 

 readily detected since the size of the photographic image diminishes with a diminution 

 in the brightness of the variable in question. Large numbers of stars of vary- 

 ing brightness have been found in this way, and a considerable number of planetoids 

 have been added to the asteroid group as a result of their presence being caught on 

 photographic plates taken at various epochs. Many new stars and comets have like- 

 wise been found by the sudden impression they have made on the photographic record 

 of a given night. These qualitative uses of the photographic plate continue to form a 

 major feature in the program of many leading observatories. 



The more precise use of the photographic plate involves the determination of the 

 exact positions of the star images on the plate and determination of the relative bright- 

 ness of the objects involved from the apparent size and brightness of the stellar images 

 represented. The branch of astronomy which has to do with the calculations of the 

 exact positions of the stars from their photographic records is known as photographic 

 "astrometry," whereas the field of investigation involved in determining the brightness 

 of the stars from the appearance of their photographic images comprises the branch of 

 astronomy knowm as photographic "photometry." 



Astrometry. — In utilizing the photographic plate for determining accurately the 

 positions of the stars represented upon it, use is made of a precision comparator, or 

 measuring machine. This machine constructed in various forms usually consists of a 

 carriage to which the negative is attached and which is capable of movement by microm- 

 eter screw^s in two directions at right angles to each other. It is of paramount 

 importance that the lead screws of the measuring machine should be of the highest 

 possible precision if great accuracy is to be expected in the results of plate measure- 

 ments. A microscope provides for a suitable magnification for the photographic 

 images, each of which is bisected by the cross wdre in the eyepiece of the microscope 

 while the scale readings of the micrometer screws are set down in turn. It is consid- 

 ered good practice to make from three to five settings upon each star image in turn, 

 alw'ays approaching the star image wdth the cross wire moving in the same direction 

 to avoid backlash. After readings have been made on all the desired objects, the 

 plate is then usually reversed 180° on the carriage for a complementary series 

 of measurements. 



The results of the tw^o series are then averaged together for the x and y coordinates 

 of each object referred to a hj^pothetical origin. The reduction of these x and y linear 

 measurements to the astronomical coordinates, "right ascension" and "declination," 

 involve fmidamental equations in spherical astronomy, corrections to the curvature 

 of the field of the telescope, ard corrections to the adopted scale value of the instru- 

 ment involved. These considerations are beyond the scope of this chapter. 



Distortion.— T\\G interpretation of the positions of stars determined from photo- 

 graphic plates raises the question of possible distortion that may be introduced through 



