218 HOW TO SEE WITH THE MICROSCOPE. 



microscope. In this tube is placed a circular glass disk 

 (one of the ordinary circular covers used in mounting* 

 object slides). This disk is supported in position by a 

 horizontal pin, to which it is fastened with a bit of ce- 

 ment, the pin passing to the outside, and terminating 

 with a little knob, by which the disk can be revolved 

 at will. The tube is pierced laterally with an aperture 

 for the admission of light. The light from an ordinary 

 hand lamp passing through the aperture is received by 

 the little glass disk, and, by turning the outside knob 

 properly, is made to pass vertically downwards to the 

 rear of the objective, and by it in turn concentrated oil 

 the object to be examined ; no sub-stage or other illu- 

 mination being used. This is the instrument as made by 

 the Messrs. Beck, and is in fact but a modification of a. 

 similar instrument invented by Prof. H. L. Smith, of 

 Geneva, N. Y. Prof. Smith's device, however, is fur- 

 nished with an interior metal reflector in place of the 

 glass disk of the instruments of the Messrs. Beck. 



We had used the illuminator but a very short time 

 when we discovered that the definition of the objective 

 was very much improved by shutting off the area of the 

 lateral aperture, thus allowing less light to enter. We 

 also found that the actual amount of light needed de- 

 pended on the objective employed. In most instances 

 the higher the angle of the object-glass, the smaller 

 should be the area of the lateral aperture. 



About the time we had arrived at the above fact, the 

 Hon. P. H. Watson happened to call in to spend an 

 hour or two " over the tube." The conversation turn- 



