1889.] MICKOSCOPrCAL JOUKNAL. 123 



to be viewed from the front, it being thrown on a piece of white paper, 

 as in Nachet's method. 



With regard to the choice of a room to serve as a laboratory for pho- 

 tomicrographic work, and the setting up and adjustment of the ap- 

 paratus, Dr. Zeiss's very elaborate catalogue of photomicrographic 

 apparatus, to be obtained from F. R. Emerich & Son, New York, 

 should be consulted, in which valuable information is also given on the 

 nature of different souixes of light and the manner of their use for pho- 

 tomicrography, and on the special precautions required in the chemical 

 part of photomicrography. 



In photomicrographic work an objective of 75 mm. focal length has 

 been constructed which serves to take large objects (2 to 4 cm.) under 

 a magnification of ten to fifteen times. It possesses all the advantages 

 of the other apochromatic objectives. 



As illuminating apparatus, either an Abbe condenser of 1.20 to 1.40 

 mm. aperture or a specially constructed achromatic condenser of 1.0 

 mm. aperture can be used. To obtain a successful photomicrograph 

 it is necessary that the' illumination should be limited to that part of the 

 object which it is desired to photograph, because otherwise the light 

 coming from the surrounding parts has the effect of fogging the picture. 

 A sharp image of the source of light must therefore be projected upon 

 the object, and to this end the condenser is provided with an arrange- 

 ment for ci-oss-centering and for fine-adjustment. The limitation of the 

 illuminating cone is effected by an iris-diaphragm. 



For the 75 mm. objective a specially small lens of great focal length 

 is used as condenser, since it is here necessary to project an image of 

 the source of light within the objective. The condenser for use with 

 the electric arc light consists of two plano-convex and one concavo-con- 

 vex lens. The part of the system near the lamp is fixed once for all at 

 the proper distance for producing a parallel beam, and to diminish 

 spherical aberration the concave face is turned to the lamp. The part 

 turned to the microscope, which brings the pai'allel rays again to a 

 focus, is movable in a sliding socket which permits the displacement 

 of the image on the optic axis within pretty wide limits. 



Note by Professor Hitchcock. — As soon as time permits him to 

 write up some notes a few words may be anticipated, especially with 

 reference to the use of plates prepared with coloring matters. The 

 Zeiss apparatus for photography is unquestionably the most perfect and 

 complete arrangement yet devised. It is expensive to be sure, but it is 

 always ready for use. About 600 marks would cover the cost of the 

 essential parts. 



If larger pictures are desired a dark room may be used and the image 

 projected through the long camera upon a screen. 



A New Apochromatic Test. — The new test just discovered is the 

 butterfly Colias ccesonia (foreign) . Those scales distinguished by fine 

 ribs widely separated are remarkable for closely-packed molecules, 

 lying in curvilinear rouleaux generally about 1-1 20000th of an inch, 

 and with the best glasses throw up brilliant focal discs. — G. W. Roys- 

 ton-Pigott in English Mechanic, Apr. 19, 1889. 



