OF THE MICROSCOPE. 



60 



probably find that amongst the combatants even the definition 

 of angular aperture seems to be undecided, all of which is, of 

 course, very puzzling to a beginner. We do not propose to 

 decide this vexed question, but the following statement will, 

 we hope, enable our readers to form a clear idea of what the 

 older writers meant by this term. 



It was Dr. Goring, we believe, that first pointed out the 

 special advantages of high angles, and suggested the use of 

 test objects, and the figures on the preceding page were used by 

 him to define and explain what he meant by angular aperture. 



In these figures, L L and L' L' are two lenses of the same 

 magnifying power, 'but different angular apertures. It will be 

 seen that the cone of rays proceeding from O, is substantially 

 the same as that from O', but that the lens LL takes in a larger 

 part of the cone from O. than the lens L' L' does of the cone 

 from O'. The angles L O L and L' O' L' are the respective 

 measures of the angular apertures of the two lenses. 



The definition of angular aperture, 

 given by Goring, has been followed 

 by all subsequent writers, the ac- 

 companying figure being that used 

 by Dr. Carpenter for the purpose of 

 explaining and defining the same 

 thing. In this figure the compound 

 lenses, 1, 2 and 3, are parts of an 

 achromatic objective without its 

 brass mounting. The line, a c, is 

 the front of the objective, and b is 

 the object. In this case the angle, 

 a b c, is the angle of aperture, and it 

 is evident that if a c had been made 

 of a larger diameter, or if the curva- 

 tures of the lenses, 1, 2 and 3, had been such that the distance 

 of a c from b had been lessened, the angle, a b c, would have 

 been increased. The points that we have mentioned as govern- 

 ing the angular aperture are in general controlled by the 

 character of the glass used, and by the formula (as regards 

 curves, distance, etc.) employed by the optician, and he cannot 

 Deviate from them without an entire change of design, 



Fig. 1G. 



