IMMERSION LENSES 119 



2. Arrangement of the object. 



The object to be examined under the microscope must be mounted on a 

 microscope slide a thin very transparent piece of glass free from bubbles 

 of air and may be covered with a cover-glass a much thinner and smaller 

 piece of glass, square or circular in shape, and measuring 18-25 mm. in 

 diameter, but not exceeding 0' 15-0' 20 mm. in thickness. 



The rays of light coming from the object as they pass through the cover-glass 

 will be displaced to a greater or less extent, depending upon the thickness of the 

 glass. Fig. 112 shows this. Given any point A on the object, its image on account 

 of displacement will appear along the line DE, and 

 will be diffuse ; with [dry] high-power lenses especi- 

 ally, much of the brightness and sharpness of the 

 image will be lost. 



[To obtain perfect definition with the higher 

 powers of the microscope, the thickness of the 

 cover-glass is important, and for two reasons : 



1. "If the cover-glass be very thick, there may 

 not be room enough to bring the front lens 

 sufficiently near to focus the specimen. 



2. " The varying thickness of the actual glass 

 introduces errors in the adjustment of the com- 

 ponents of the lens system (Spitta).] 



To overcome this difficulty, it is only necessary glass. 

 to use cover-glasses of the thickness indicated on 



the objective, each objective being corrected to work for a given thickness. Or, 

 since cover-glasses of exactly the same thickness cannot always be obtained, one 

 may have objectives of certain magnifications, which can be corrected by altering 

 the distance between the component lenses : the thicker the cover-glass the nearer 

 must the lenses be together. 



But now that all microscopes have a draw tube this correction is really not of 

 vital importance, because by altering the length of the tube the effect of the thick- 

 ness of the cover-glass can within certain limits be counteracted. The thicker the 

 cover-glass the shorter must the tube be. With the draw tube right down in its 

 socket, cover-glasses 0'25 mm. thick can be used, but with a normal length of tube 

 (160-170 mm.) one must have cover-glasses no thicker than 0'15 to 0'18 mm. 



3. Homogeneous immersion lenses. 



Immersion lenses are used in order to counteract the refraction of rays 

 of light in passing from glass to air. In using an immersion lens a drop of 

 some liquid, the refractive index of which is as nearly as possible the same 

 as that of glass, is placed on the cover-glass, and the lens lowered into it. 

 Cedar- wood oil has a refractive index of T515 to 1'520, a mixture of castor- 

 oil and essence of anise about 1-510, and monobromonaphthaline T66. 

 Homogeneous immersion objectives do not need to be corrected. 



When rays of light pass from the cover-glass into air, their direction is altered 

 in such a manner that all rays making with the surface of the cover-glass a smaller 

 angle than [48 12'] are totally reflected and are lost to the objective. By sub- 

 stituting a substance of the same refractive index as glass for air, this loss of light 

 is avoided. An immersion lens makes the image very much [brighter and] 

 sharper ; so that an homogeneous immersion objective whose angle of aperture 

 measures 82 has the same value (i.e numerical aperture) as a dry lens whose 

 angle of aperture is 180 (//, sinU) (p. 112). Moreover for the same magnification 

 an immersion objective has a greater focal length than a dry lens. 



It is necessary to use an Abbe condenser with an immersion lens, and 

 perfect results can only be obtained with a given length of tube (generally 

 160-170 mm,). A drop of cedar-wood oil is placed on the cover-glass, and the 

 objective is lowered until its front lens touches the oil. 



