16 



THE MICROSCOPE 



cover glass. 



of body. To obtain the most perfect results a tube length of 

 160 mm. should be used — the drawtube of the microscope is 

 graduated, and can be set at this figure. If a revoWng nosepjece 

 is. in use, this lengthens the body 15 mm., and the drawtube should 

 be set at 145 mm. instead of 160 mm. ; with a Sloan object glass 

 changer measuring 10 mm. it should be set at 150 mm. 

 Thiokneas ot The tMckness of the cover glass used over the object has no 

 effect with an immersion lens and but slight influence with the 

 low powers, but is a matter of importance with a high-power 

 dry lens. A 1/6-inch object glass can only be optically correct 

 for one thickness of cover glass, and it is most important to 

 always use those known as No. 1 thickness. The object 

 glasses, unless otherwise ordered, are always made for a thickness 

 of "007 inch ('18 mm.), which is the average thickness of No. 1 

 cover glass. Thicker cover glasses should only be used for 

 objects to be examined with low powers. 



The delineation of fine structure depends upon the aperture 

 of the object glass being sufficiently large to produce an image 

 of this fine structure, but combined with this it must possess a 

 sufficient degree of magnification to enable this image to be clearly 

 seen. We may know that the finest lines of an etching or steel 

 engraving exist in a print, but it may be necessary to magnify 

 the image in order to make them visible as single lines to the 

 eye. If the print is magnified further, the fine lines appear 

 thicker, but no further fine lines are there to be seen. Thus 

 lines which are invisible require a certain degree of magnification 

 to see them clearly, but extra magnification beyond this point 

 is useless. So with a microscope object glass, it must possess 

 a large enough aperture to produce the detail in the image, and 

 the magnifying power need not be more than enough to enable 

 the eye to see it clearly. 



Each object glass has a particular aperture, sufficient to 

 form an image of all the detail that can be seen with the magnify- 

 ing power given by it in conjunction with a moderate eyepiece. 

 The following table gives the apertures of standard object glasses : 



Apertures 

 suitable for 

 different 

 powers. 



Table of 

 apertures 

 and powers, 



