1890.] MICROSCOPICAL JOURNAL. 263 



Experiment 7. — Delicate imperfections in the lines of an Abbe test- 

 plate, under a Zeiss J-inch objective and a two-inch eye-piece, on a hori- 

 zontally arranged No. 3 Powell and Leland stand, were focussed for a 

 normal eye looking through the microscope. A sharp, real image 

 could not be seen in any plane within six feet of the eye-piece. A four- 

 drachm weight was carefully placed on the cap of the eye-piece, when 

 a sharp image of the imperfections appeared on the screen about fifteen 

 inches from the eye-piece. On again looking through the microscope, 

 the imperfections of the virtual image had lost their sharpness. The 

 weight acted on the eye end of the microscope tube as on a lever and 

 thus slightly tipped up the objective and increased the distance between 

 it and the object. 



Experiment 8. — The dark dots on a light ground of plieurosigma an- 

 gulatum^ under a Powell and Leland j^s-inch apochromatic and a one- 

 inch compensating eye-piece, on a horizontally arranged No. 3 Powell 

 and Leland stand, were focussed for a normal eye looking through the 

 microscope. On removing the head, a blurred image appeared on the 

 screen about fifteen inches from the eye-piece, but this blurred image 

 presented the reverse appearance of light dots on a dark ground. A 

 one-drachm weight was carefully placed on the cap of the eye-piece, 

 when a sharp image of the dark dots on a light ground appeared on the 

 screen. On again looking through the microscope, the dark dots had 

 lost their sharpness. By removing the weight, the original conditions 

 werfe restored. The sharp black dots were once more seen through the 

 microscope, and only blurred white dots on the screen. 



Inductions from these experiments are : First, wdien the microscope 

 is focussed for a normal eye on an object approximately a single plane, 

 the instrument does not project a real image of the object above the eye- 

 piece ; and, secondly, when the microscope is so arranged as to project 

 a real image of a very thin object on a screen above the eye-piece, 

 the object is not in focus for a normal eye looking through the instru- 

 ment. These inductions are in harmony with theoretical optics. Ob- 

 servations really opposed to such inductions must fail to find supporting 

 explanations in theoretical optics. 



The announced chief purpose of this paper, to record a series of ex- 

 periments for particular ends, has been attained. A secondary purpose 

 is to call attention to the fact that the recorded experiments also at least 

 suggest explanations of the apparently opposed phenomena brought to 

 the notice of the societv a year ago. 



Let us consider Dr. Blackham's apparently contradictory demonstra- 

 tion. The object was a section of animal tissue, showing well injected 

 blood-vessels, and, therefore, necessarily somewhat thick. A normal 

 eye looking through a microscope might focus the nearer surface of 

 such a section, and on removing the head find a real image of a deeper 

 plane projected on a screen above or beyond the eye-piece. In this 

 instance the virtual and real images would be in a general way alike, and 

 might be carelessly considered images of one and the same plane of 

 the object. I believe those of us who were present and had glimpses 

 of Dr. Blackham's virtual and real images were careless in the way 

 suggested. 



In regard to the fact that some members had focussed an object look- 

 ing through the microscope and then without changing the conditions 



