448 THE MICROSCOPE. 



instrument. Such an illuminating lens should be arranged with a sys- 

 tem of both central and external stops, each revolving on a separate 

 axis ; and an adjustable cap to slide over the top of the lens, containing 

 a crystal of herapathite mounted between thin glass; also a plate of 

 selenite mounted in the same way, to use on the stage above it. Ob- 

 jects of too great density for transmitted light will appear under this 

 mode of illumination as if in relief ; and the definition of the various 

 parts will be so accurately displayed as to constitute a most perfect 

 method of viewing them. 



Before we conclude, we beg to direct the attention of the reader 

 to the beautiful experiments of Mr. John Gorham,* as illustrating the 

 wonderful magnifying power of the human eye. The fact itself is 

 one of great interest to the microscopist, who has hitherto almost ex- 

 clusively availed himself of the optician's aid in his examination of 

 minute bodies. 



We are only able to notice the results arrived at by this able in- 

 vestigator, which are as follows : " That when small bodies are brought 

 very near to the eye, their images are magnified, just as images of 

 larger objects, when seen at a distance, are diminished, and by the same 

 law. The apparent magnitude of objects depends on their visual angle. 

 The visual angle, for short distances, may be well illustrated by em- 

 ploying a small circular disk of light. This minute circular disk of 

 light is procured by perforating a card with a needle. A sewing- 

 needle, of the size marked No. 7, produces an aperture about the one- 

 fortieth of an inch in diameter. In order to examine the light which 

 is transmitted through such an aperture, all extraneous rays should be 

 excluded ; hence the plane in which the opening is made should be 

 placed at the end of a tube. The pencil of light admitted through an 

 opening of this kind, held within an inch or so of the eye, consists of 

 rapidly-diverging rays falling upon the cornea. Some of these are 

 entirely lost, others are intercepted by the iris, while the remainder 

 pass on through the pupil, which communicates to the image formed 

 on the retina its circular form. 



For the purpose of presenting very small objects mounted on 

 microscopic slides in the usual way before the eye, and to exclude the 

 surrounding rays of light, take an upright box of pasteboard about one 

 inch and a half deep, and an inch and a quarter in diameter (two pill- 

 boxes joined will do very well); cut a couple of slits through one of its 

 sides, sufficiently large to admit of the slips of glass sliding to and 

 fro. Make two apertures (perforations with a needle) opposite to each 



* Published in the Quarterly Journal of Microscopical Science, October 1845. 



