20 THE PRESIDENT'S ADDRESS. 



place the tinfoil on a smooth piece of cork and make several 

 fine punctures in the foil, then placing the foil in a compressor 

 view it under a f objective. When the image of the edge of 

 the flame is focussed by the substage condenser on the foil 

 the only image that can be obtained will be that of the 

 diaphragm at the back of the condenser. The image of this 

 will be erect when the hole in the foil is beyond the focus of 

 the objective, and inverted when within the focus of the objec- 

 tive. A better plan, however, is to rack the condenser so that 

 its focus shall be either within or without the plane of the foil, 

 because it will then be possible by altering the focus of the 

 objective to obtain an image of the edge of the flame itself. If 

 an opaque object, such as the flat of a paper-knife, be moved 

 up and down in front of and near the flame it will then at once 

 be seen whether the image is erect or inverted by watching the 

 direction in which the image of the flame is extinguished; 

 thus, if the paper-knife be moved upwards, and the image of 

 the flame is extinguished in a downward direction, the image 

 is an inverted oue, and vice-versa. The results obtained by the 

 above method are given in the following table : — 



We see, therefore, that when the foci of both the condenser 

 and the objective are either beyond or within the plane of the 

 object the image will always be inverted, aud that when one is 

 within and the other beyond it will always be erect. 



Now, if we examine other structures, such as the eye of a fly 

 or beetle, diatomic structures, dry or in balsam, the bubbles in 

 the fluid cavities in quartz sections, or the fluid cavities them- 

 selves, we shall always find that the images will follow the rules 

 of those of minute perforations in an opaque substance as given 

 in the above table. It is difficult, therefore, to see how the 

 character of the image can decide the question as to whether 

 the minute structure under consideration is acting as a positive 

 or negative lens. 



In the first paragraph of the appendix Mr. Allan Dick men- 

 tions a difficulty which has probably puzzled not a few. He 



