INTRODUCTORY REMARKS. 41 



microscope is chiefly dependant upon the angle of aper- 

 ture of its object-glass, (freed from the aberrations just 

 mentioned), it is of some importance to be able to mea- 

 sure this angle with accuracy. When the object-glass 

 consists of a single lens, or of two cemented together, 

 its angle of aperture may be easily determined by the 

 actual measurement of its acting focal length and 

 diameter. But if it consist of two or three pairs of lenses, 

 not in contact with each other, this mode of measure- 

 ment will not give a correct result. The simplest and 

 best method in such cases, and indeed I may say in all 

 cases, is by means of the Microscope-gnomeometer, de- 

 scribed at page 218 of the Microscopic Cabinet. 



Considering what has been just stated, I proposed, in 

 1835, that achromatic object-glasses should be applied to 

 the micrometer microscopes which were made use of for 

 reading off the divisions whilst making the trigonome- 

 trical survey of Ireland. They have since been used by 

 astronomers for similar purposes in the microscopes 

 attached to transit circles ; the advantage of which is, that 

 observations can be read off much longer in the twilight 

 of an evening without artificial illumination ; thus caus- 

 ing less fatigue to the eye than is experienced by looking 

 alternately at objects illuminated by different coloured 

 lights. 



Lest it should be imagined that I have dwelt somewhat 

 too much upon the subject of angular aperture, I will ad- 

 duce a case in point to shew the necessity of its being 

 better understood than it is. I have in my possession at 



