24 



VEGETABLE HISTOLOGY. 



a system of lenses is almost free from aberration defects. 

 According to the method adopted by the maker, objectives are 

 designated by letters, as A, B, C, etc., or by numbers, as 1, 2, 3, 

 etc., or by figures which represent focal lengths. In the latter 

 method, which is the most rational, if an objective is marked, 

 say 1 inch or 2 / 3 inch, this means that its magnifying power is 

 the same as that of a simple lens whose focal length is 1 inch or 

 2 / 3 inch. In order to know which is high or low power, the stu- 

 dent should remember this rule : The smaller the number or 

 fraction representing the focal length of an objective, the greater 

 is its magnifying power. The same rule applies to oculars or 

 eye-pieces. The objectives mostly used in vegetable histology 

 are those of 1 inch, 2 / 3 inch and l / fl inch focal length. The dis- 

 tance between the front lens of the objective and the object 

 when in focus is about equal to the focal length of the objective, 

 and is known as the working distance. 



Objectives are either dry lenses or immersion lenses. If, as 

 is usually the case, there is an air space between the objective 

 and the object, the lens is called a dry one ; if a liquid is between 

 the object and the lens, the lens is called an immersion lens. 

 The liquid may be water or an oil. If water, we have a ivater 

 immersion lens; if oil, an oil immersion lens. If the index of 

 refraction of the oil is about the same as that of glass, we have 

 a homogeneous immersion lens. Cedar oil thickened by evap- 

 oration is an example of such. Lenses intended for immersion 

 must be constructed accordingly. The great advantage of im- 

 mersion is that the angle of the cone of light that can be util- 

 ized by the lens is considerably increased, thereby increasing 

 the illumination and the efficiency of the microscope. 



Fig. 24. 



Fig. 24 represents the construction of two dry objectives 

 (3 and 6) and an oil immersion objective ( 1 / 12 inch focal length) . 

 Piece 3 has % inch focal length and consists of two compound 

 lenses or doublets, in which concave and convex lenses are com- 

 bined. Piece G has one single lens and two doublets and a focal 



