On the Formation of the Paraboloid. 187 



In no way have I ever seen minute organisms or animalculae so 

 beautifully displayed as in this. The minutest details are visi- 

 ble in their true colors on a black field. I am not aware that 

 any one has provided himself with this piece of apparatus, and 

 the knowledge of its effects probably does not extend beyond 

 some half dozen friends that have seen my demonstration; it is, 

 perhaps, destined to be announced as a new discovery. 



The so-termed immersion paraboloid has recently been 

 claimed in this way by Prof. Edmunds, who has attached his 

 name to it — not quite in ignorance of what I had previously 

 done; for in the only interview that I ever had with him, he in- 

 sinuated that I had made the focus fail within the body of the 

 glass. I had previously made various experimental paraboloids 

 up to three inches in diameter, with the result that I did not 

 find that the effect was increased by the diameter of the annu- 

 lus of light, but quite the reverse. The most perfect form of 

 immersion paraboloid would be of smaller size, to be used with 

 objects mounted on thin glass. It is always an easy matter to 

 condense light by extraneous means, so as to obtain any re- 

 quired degree of intensity. 



It is to be regretted that in this country the noble art of me- 

 chanical construction should be held in such low esteem as not 

 to be considered a worthy element of education, enabling per- 

 sons to carry their own ideas into practice without being ham- 

 pered, or altogether stopped, by heavy artisan's bills. 



The parabola is a definite geometrical curve, and the only 

 variation that can be made is one of focus. In all that I have 

 yet constructed, I have not wasted my time with such a blun- 

 der as working anyone of them with the focus uselessly within 

 the glass. 



This statement leads to the inference that information relat- 

 ing to practical methods of obtaining parabolic forms may be 

 useful to those engaged on the subject. My descriptions must 

 be taken as interpretations divested of the ever recurrent letter- 

 ing, under which geometrical demonstrations are usually 

 smothered. 



The glass parabolic illuminators are ground up to form by 

 means of templates. These may be accurately formed by a 

 purely mechanical method, based on the principle that every 

 section of a cone taken in a plane parallel to the opposite side 

 is a parabola. Proceed as follows: Turn a cone, either of metal 



