634 



NOTE. 



On the Use of the Esculin Screen in Photomicrography. 



By Fkederic E. Ives. 



1 believe I was the first to point out, about fifteen years ago, in 

 a verbal communication to the Photographic Society of Phila- 

 delphia, the advantage to be gained in photomicrography with 

 ordinary achromatic objectives, by employing screens to cut out 

 those spectrum rays for which the objectives are not well corrected, 

 and presented a collection of examples obtained with colour- 

 sensitive plates and colour screens, of a quality which could not 

 otherwise have been obtained with the objectives employed. 



Since then, I have always, until quite recently, employed a 

 yellow screen and orthochromatic plates for photomicrography 

 with achromatic objectives, though preferring to use apocbromatic 

 objectives for high-power work, because they are corrected for rays 

 which resolve finer details, and also permit the use of transparency 

 plates, which give brilliant and fine-grained negatives. 



Inasmuch as most achromatic objectives appear to be well 

 corrected for the bright blue spectrum rays, it has occurred to me 

 that transparency plates might be satisfactorily employed with 

 them if the action of the violet and ultra-violet rays was entirely 

 suppressed. 



To test the relative effect of the blue and ultra-violet rays, I 

 have employed a combination of Bausch and Lomb § achromatic 

 objective and 1 in. huyghenian eye-piece, which gives an ex- 

 cellent image to the eye, but works very badly with daylight on 

 ordinary photo plates. 



One exposure was made through a dry esculin screen as recently 

 described by me in " Camera Work," July 1904, p. 44, and another 

 through a uranine screen ; both with diffused daylight. 



The esculin screen suppresses the ultra-violet and part of the 

 violet spectrum rays ; and the photograph, made chiefly by the 

 blue rays, is a very good one. The uranine screen suppresses the 

 blue spectrum rays and part of the violet ; and the photograph, 

 made chiefly by the ultra-violet rays, is very bad indeed. The 

 photographs are submitted herewith, and the conclusions to be 

 drawn are sufficiently obvious. With no screen, the result is inter- 

 mediate in quality between the two examples shown, but is bad. 



Achromatic objectives used without an eye-piece sometimes 

 work very well on ordinary plates, but I have yet to find one that 

 does not give distinctly better results with the esculin screen.* 



* Mr. Ives kindly sent two photographs which bear out the statements in the 

 foregoing note; one was sharp, the other quite woolly. — Ed. 



