L899J MICROSCOPICAL JOURNAL. 117 



magnification is close enough lor ordinary purposes. If 

 this plan is tried with high powers, the illumination is 

 feeble, and the light must be shaded from the image — 

 either by putting the microscope and lamp in a bo'i 1 / with 

 a hole for the tube, or by using a camera, with its lens 

 removed, and receiving the image on the ground glass, 

 the head being under a focussing-cloth. For these pow- 

 ers, however, a close enough approximation can be made 

 by comparison. Say, for instance, you have found the 

 focus of one lens, and it is 1-2 in. If this lens magnifies 

 with an eyepiece 100, this will give 5 as the magnification 

 of the eyepiece. Say another lens magnifies with the 

 same eyepiece 500, then this gives 100 as the magnifica- 

 tion of the objective, and the lens is 1-10 in. focus. As 

 for measuring the magnification of a microscope, Beale's 

 camera is the simplest, and easily made. Gret a pill-box, 

 or make a cardboard tube to fit over the eyepiece of the 

 microscope. Cut one end of the tube to an angle of 45°, 

 and fasten by means of cement a thin cover-glass over it, 

 and over this make a hole about 1-4 in. diameter, and 

 blacken the inside. To use it, place the microscope hori- 

 zontally and at 10 inches from the table, and then you can 

 easily draw anything on the stage. If a stage microme- 

 ter is placed there, you can draw the magnified image, 

 and thus know the magnification. If 1-100 in. measures 

 one- inch, the magnifying power is 100 diameters. 



C. R. Cross on Focal Length. 



Cross's Formula. — Dr. J. J. Woodward, of the U. S. 

 Army Medical Museum, published in the American 

 Journal of Science and Arts, for June, 1872 : "Remarks 

 on the Nomenclature of Achromatic Objectives for the 

 Compound Microscope" in which he said: For some 

 years past, while most of the Continental opticians have 

 contrived to give arbitrary designations to their ach.ro- 



