Reade, on a New Hemispherical Condenser. 61 



under this illumination, though with direct solar light I 

 had no difficulty whatever. 



It is probable that a similar application of the hemisphe- 

 rical lens and central stop to the oxyhydrogen microscope, 

 which our variable climate almost compels us to use, would 

 in like manner throw into the pictures on the screen the 

 additional charm of natural colours, and thereby greatly en- 

 hance the interest of the exhibition. 



Notwithstanding my use of the condenser in the experi- 

 ments just described, it did not occur to me to extend 

 the application of it, until I was, as it were, driven by 

 necessity. My old parishioners and other kind friends pre- 

 sented me with a valuable microscope at the commencement 

 of last year ; and not having, in the first instance, any of 

 the well-known condensers at hand, I used the light of two 

 lamps placed at right angles to each other, and by means of 

 suitable lenses I threw sufficient light on the rectangular 

 markings of the P. acuminatum and other similar tests. I 

 was much pleased with the eflFect of this simple method of 

 illumination ; and I am glad to find that Mr. Tomkins has 

 also used it, but with considerable improvement, by employ- 

 ing two achromatic prisms, which give brilliant illumination, 

 while the " marking shadows'^ are in deep relief. 



In order to obtain any proper definition of the markings, 

 I found it necessary so to turn the valve of the diatom, that 

 a line of markings might lie at right angles to a line of light. 

 In fact, in any other position the markings are scarcely 

 visible ; and the conclusion seemed forced upon me, that the 

 ordinary spot lens contains in its circle of light a large 

 portion of unnecessary, if not injurious, illumination. With 

 this impression on my mind, it suddenly occurred to me, that 

 my old friend, " the kettle-drum,^' as Mr. Gravatt calls my 

 condenser, might play an important part, if its plain surface 

 were covered with tinfoil suitably pierced at the circum- 

 ference for the tranmission of two pencils of light at right 

 angles to each other. I made the experiment, and happily 

 I can fall back upon the testimony of well-qualified ob- 

 servers as to the success which attended it. The direct illu- 

 mination of only one lamp was now sufficient, and, instead 

 of rotating the object — always a difficult process in the 

 absence of suitable adjustments — it was easier to rotate the 

 secondary stage which held the condenser, and so gain the 

 proper position of the two points of light. It may be well 

 to state, that by taking out the eye-piece, and looking at the 

 points of light down the body of the tube, we may at once, 

 by the rotation of the sub-stage, place them in the right posi- 



