

RULES FOR INVENTION. 107 



scope, by the late Mr. G. Adams, and ourselves, where the 

 diminution of light was of less consequence than indistinctness 

 of the image In many cases the combination of two convex 

 lenses answer very well : but the combining of-two similar plano- 

 convex lenses together, of superfluous diameter and thickness, and 

 for the greatest defect or aberration in the worst position to each 

 other; and afterwards to palliate it with a small aperture as 

 shewn in figure 4, is such an anomaly or absurdity in opitics as 

 not to require any serious comment on my part. I shall only 

 appeal to the least experienced constructor of microscopes, 

 whether he does not know, that the substitution of a double 

 convex lens of the diameter only of Dr W.'s aperture, and of 

 the s:ime focus, would produce an image infinitely more perfect 

 and vivid than the mutilated one proposed by Dr. W. 



From these remarks I presume there will be nothing to 

 apprehend from the attempt of Dr. W. to depreciate the 

 excellence of the spectacles, Camera Obsuras, and Mocro- 

 scopes, as have been constructed by the most eminent Opticians 

 of the day. 



I am, Sir, 

 » Your's, &c. 



W. JONES. 



Holborn, \6thJan. 1813. 



IV. 



Rules for discovering new Improvements, exemplified in the art 

 of thrashing and cleaning grain ; hulling rice ; warming rooms ; 

 preventing ships from sinking, &c. By Oliver Evans, of 

 Philadelphia*. 



NECESSITY is called the mother of inventions j but upon origin of In. 

 inquiry we shall find thatreason and experiment bring them ventions. 

 f or th j for almost all inventions have been discovered by such 

 steps as the following -> which may be taken as a 



* From the Appendix to his" Young Mi!l-wright and Miller's Guide" 

 printed by subscription in Philadelphia, but very sarce in this country. 



