no THE MICROSCOPE 



The second type of instrument therefore, while giving good 

 resolution, was not free from other defects, and was not there- 

 fore equal to the monocular. 



The third type of binocular, which consists of two 

 microscopes set at an angle to one another, both pointing at 

 the focal point, while quite saftisfactory in its performance, is 

 limited to the use of low powers and requires specially mounted 

 and accurately adjusted pairs of object glasses. 



It will be gathered from this description of the properties of 

 previous instruments what were the difiSculties that had to be 

 overcome in making a really satisfactory binocular, and in the 

 following description of the properties of the Beck binocular we 

 treat each point separately, explaining how the objections have 

 been removed. 

 Beaointiou. The resolving power of a microscope is a measure of the 

 fineness of detail that it will depict in the image which it forms, 

 quite apart from the magnifying power. The microscope must 

 have sufficient magnifying power to render such detail visible to 

 the eye, but no amount of extra magnifying power is of use 



unless the resolving power is 



sufficient to produce an image 



containing the requisite detail. 



Resolving power depends upon 



the size of the cone of light 



Fig. 103. which forms each point of the 



image. Suppose the lens 



(Fig. 103) represents the object glass forming an image of the 



central point of the object C at a point D in the centre of 



the image; the resolution for a given magnifjdng power will 



depend on the diameter A B of the cone of light A D B which 



forms the image ; this cone of light has an exact ratio to the 



angle A C B of the light which enters the lens from each point 



of the object, and it is by means of the angle A C B that 



the resolving power is generally and more conveniently expressed 



as numerical aperture (N.A), but it might be expressed with 



reference to the angle A D B, It is evident that if the cone 



of light A D B be bisected and the complete half D A be 



used to form the image received by one eye and the complete 



half D B used to form the image received by the other 



eye, the cone of light forming each image is only half the size, 



and the resolution or power of depicting fine detail is reduced 



thereby ; thus this method of making a binocidar microscope 



reduces its power of resolving fine detail. 



The new Beck binocular acts on a different principle. Above 

 the object glass is a prism shaped as shown in Fig. 104. The 

 whole of the light from the object glass passes through the 

 surface of the glass B A to a surface E A, which is coated with a 

 semi-transparent surface of silver. This allows part of the light to 



