THE DEVELOPMENT OE THE PHOTOGRAPHIC OBJECTIVE 51 



distance of the object is then marked on the lens mount, and the auxihary lens becomes 

 a "portrait attachment." 



If a negative lens is added to an existing lens, the focal length is increased, and a 

 larger picture results. The original Dallmeyer Adon lens was an interesting extreme 

 case of a supplementary lens, for the Adon was a 2 X Galilean telescope system ; if the 

 telescope were focused correctly for infinity, it would serve to double the size of the 

 image of a distant object without upsetting the focus adjustment of the camera. 



Astronomical Photographic Lenses.— At first, astronomical photography was done 

 by merely placing a photographic plate in the focal plane of an ordinary telescope. 

 However, in such a case the field of view is very small, being only about a degree in a 

 refracting telescope and but a few mmutes of arc in a reflector. Also, in a refractor 

 the visual achromatism is entirely unsuited to photographic work, and a chromatically 

 undercorrected zero-power "correcting lens" is commonly inserted in the telescope 

 tube to improve the achromatism for photography. 



As soon as photographic materials of a suitable character became available, 

 toward the end of the last centurj'^, astronomical photography rapidly became more 

 and more common. To cover a wide field of sky in a single exposure, large photo- 

 graphic lenses of the Celor or Cooke types were used. These have been constructed in 

 focal lengths of several feet, at apertures up to //6 and are very highly corrected 

 s3^stems. 



In recent years, a number of interesting small lenses of great relative aperture have 

 been developed for astronomical purposes. The first was Dennis Taylor's //2 lens^ 

 consisting of two similar cemented trip- __„ 



let combinations, together with a strong ^^^ I 



concave lens placed very close to the 

 focal plane (Fig. 70). This concave lens ~ 

 acts as a "field flattener," according to 

 the plan suggested in 1866 by Piazzi- 

 Smyth. By virtue of its position, the Fig. 70. — Taylor //2 Astro lens, 



concave lens has practically no effect on 



the focal power or on the spherical aberration of the system, but it carries its full 

 weight in reducing the Petzval sum. The Piazzi-Smyth lens has also been applied 

 to large reflecting telescopes by Ross to improve their'field of view. 



In 1934, Rayton of the Bausch and Lomb Company constructed a high-speed 

 spectrographic objective for Mount Wilson observatory, which consisted of a 4-mm. 

 microscope objective enlarged eight times so as to give a focal length of 32 mm. The 

 numerical aperture of the microscope objective was 0.85, giving an /-number of 

 1/(2 X 0.85) = 0.59. Even this verj^ high speed was surpassed in 1936 by Bracey^ 

 who similarly enlarged an oil-immersion microscope objective having a numerical 

 aperture of 1.4. This gives an equivalent /-number of 1/(2 X 1.4) = 0.36 and repre- 

 sents an increase in speed of 2.7 times over the Rayton lens. The objections to this 

 lens are, however, the extreme smallness of the spectrum produced by it, since its 

 focal length is only 16 mm., and also the necessity of oiling the plate to the back of the 

 lens. Nevertheless, it represents the ultimate limit which has been achieved in the 

 effort to obtain speed in a photographic lens. 



The Principal Types of Photographic Lenses. — The following list includes most of 

 the lenses made hj some of the more prominent manufacturers in recent years. The 

 data of current lenses ha^^^e been derived from the makers' catalogues and advertise- 

 ments in photographic journals, and from information supplied by them. The 

 properties of obsolete lenses have been drawn largely from the bibliography given 



1 Brit. Pat. 127058. 



2 Aatrophys. J., 83, 179 (1936). 



