264- THE AMERICAN MONTHLY [November, 



of the instrument had secured a photo-micrograph of the object beyond 

 the eye-piece, the following suggestions are offered in explanation : The 

 virtual focussing may have been on the nearer surface of the object and 

 the photo-micrograph a picture of a deeper plane ; or, the observer's 

 eye may have been presbyopic or otherwise hypermetropic ; or, in the 

 case of high-power work in making a connection between the micro- 

 scope and camera a little weight on the eye end of the microscope tube 

 may have made the necessary alteration in focus ; or, a virtual image 

 may have been focussed with an erect tube when the weight of the 

 eye-piece tends to press the objective toward the object and the photo- 

 micrograph secured with a horizontal tube when the weight of the 

 eye-piece tends to tip the objective up and away from the object ; or, 

 and finally, in the case of an objective of faulty correction (and Mr. 

 E. Bausch says no ordinary objective would be so faulty) the paths of 

 the visual rays may have been so different from the paths of the actinic 

 rays that the former may have given a virtual image and the latter a 

 real image under the same conditions of the microscope. 



Procuring Amoeba for the Laboratory. 



By C. B. ATWELL, 



EVANSTON, ILL. 



A pimple method of procuring amM:)a in abundance for the use of 

 classes in biological work, has been used in the laboratory of North- 

 western University, and it may prove helpful to others, at least along 

 the Great Lakes, and to those who have been accustomed to consider 

 amoeba not readily obtainable. 



Instead of following the directions laid down in the hand-books to 

 look for them '' in standing water upon the leaves of submerged plants 

 or in the mud and ooze at the bottom," they may often be found upon 

 the algae of Lake Michigan and presumably of other lakes. 



Put a quantity of the common alga, Cladophora canalicularis in a 

 tumbler of water. Let it stand on the laboratory table for six or eight 

 days. • There will usually be present upon the surface of the mass of 

 Cladophora a thin white film or ooze, which teems with amcebae and 

 other protozoans. The alga mentioned grows along the water-line, 

 where there is the greatest agitation of the water. It can also be relied 

 upon when fresh to furnish Vorticella^ Epistylis^ and occasionally a 

 green hydra. 



It is frequently possible in the rich supply of amcfibae thus obtained, 

 to observe six, eight, or ten in the field at once. So abundant were 

 they in some material lately examined, that a large number of outline 

 drawings were made of different individuals by a score of students, and 

 micrometric measurements w^ere taken of the rate of locomotion. The 

 average length when pseudopodia were 'withdrawn, seemed about 25 

 micromillimeters. A rate of onward progress of one micromillimeter 

 per second was maintained for several minutes in several instances. 

 At the same rate, constantly maintained, the amoeba would require 1 1.5 

 days to travel the length of a meter. 



