PENETRATING POWER. 75 



in all that appertains to the microscope. Mr. Baker has 

 also made considerable advances in the same direction: 

 his objectives have been made with a greater angle of 

 aperture than formerly, and with more penetration. Much 

 misconception, however, exists with regard to the available 

 angle of aperture for optical requirements ; it will be as 

 well to remark that an admirable method of determining 

 this was proposed by Professor Govin, of Turin, which 

 consists in placing the microscope perpendicularly to any 

 plane dark non-reflecting surface (as a table covered with 

 a green cloth), and having converted the instrument into 

 a telescope, by placing above the eye-piece a suitable com- 

 bination of two lenses (such as the Examining-Glass of 

 Mr. Eoss), and then examining and marking the greatest 

 lateral distance on either side at which a clear image of 

 some distinct object, such as a narrow strip of white card- 

 board or paper, laid on the table can be perceived. " Half 

 the distance between these two points, divided by the ver- 

 tical distance of the focal point of the objective from the 

 surface of the table, will, by reference to a table of natural 

 tangents, give half the required angle of aperture. This 

 will, in many cases, be found to be considerably less than 

 what may be termed the angle of admission of diffused 

 light. To illustrate this, supposing the focal point to be 

 at a distance of O'Ol inch from the surface of the objective, 

 a reference to a table of natural tangents will show that an 

 angular aperture of 17 will necessitate a linear aperture of 

 0-22 inch : an aperture of 170 will require 0'28 inch, and 

 one of 174, of 0'38 inch, in order to admit the extreme 

 rays, which, for objectives of ^th inch focus, is manifestly 

 impossible." 



In regard to angle of aperture, it may be stated once 

 and for all, that large angle of aperture is necessarily 

 incompatible with that far more generally useful quality 

 of penetration. Penetrating power l is synonymous with 



(1) Penetrating Power. " The origin of this term will be found in the Phil, 

 frans. for 1800, in an article by Sir Wm. Herschel, entitled ' On the Power of 

 Penetration into space possessed by Telescopes.' In that article, we are told 

 that when, owing to the darkness, a distant church steeple was invisible, a certain 

 telescope described, clearly showed the time upon the clock. This, adds Sir 

 William, was not owing to magnifying power alone, for the steeple could not be 

 discerned by the naked eye. And he has shown in the same article, that the 



