AND THE MICROSCOPE. 27 



could we see him, the smaller he appeared, till at last a 

 pin's head held before the eye would have been the larger. 

 While we here remark how an object well known to us 

 becomes gradually smaller, and at last totally disappears, 

 we become aware of the means by which we can enlarge 

 an object, so as to see it more distinctly and distinguish 

 a greater quantity of separate parts in it, namely, by bring- 

 ing it nearer to the eye. 



Experiment shows us, surely enough, the applicability of 

 this means ; but we soon discover that there is a certain 

 limit here, beyond which the object cannot be brought, in 

 approaching it to the eye, without the loss of all distinct 

 vision. The cause of this lies in the construction of the 

 little camera obscura, which we call the eye-ball. This, 

 like the similar instrument of the optician, can be adapted 

 only to certain distances, and if we wish to look more 

 closely at anything, we must make a corresponding altera- 

 tion in the optical apparatus, which is done very simply, 

 by bringing before the eye a transparent body, shaped 

 according to a certain law ; and for this purpose we gene- 

 rally use glass, ground into a particular form. Such a 

 glass is a lens or simple microscope, and its action consists 

 in enabling us to see distinctly an object placed in such 

 close proximity to the eye, that it would otherwise be 

 invisible. It is unnecessary to enter into an explanation 

 of the optical laws by which this action is regulated. I 

 will only observe, that they render it very easy to determine 

 how much an object must appear to be enlarged by a 

 simple microscope of this kind. It is assumed, that on an 

 average, the human eye sees distinctly at the distance of 

 ten inches, but not when the object is brought nearer. 

 Now, if I use a glass which permits me to see the object 

 distinctly at a distance of five inches, it appears twice as 



