OF THE MICROSCOPE. 21 



tance of the two lenses from each other is increased, the dis- 

 tance at which the combination is placed from the object must 

 be made less and vice versa.* 



The early forms of the compound microscope consisted of 

 little more than the two lenses we have just described, but the 

 modern instrument, even in its simplest form, is a vastly more 

 complicated arrangement. In the best forms, for the lens next 

 the eye there is substituted an eye-piece consisting of two lenses 

 with a diaphragm between them, while the objective, or lens next 

 the object, is composed of from four to ten different pieces of 

 glass, forming two or more lenses, which are so arranged that 

 each shall correct the defects of the others, and this optical 

 combination is mounted on a stand which is sometimes a mar- 

 vel of mechanical ingenuity. 



*The student who possesses a little mechanical genius and a desire to 

 become experimentally acquainted with the properties of lenses and the 

 construction of the microscope, would do well to procure a couple of cheap 

 lenses, say one of half inch focus, and one of about two inches focus, and test 

 by actual trial the statements made in the text. Such lenses may be con- 

 veniently arranged in a tube formed of writing paper and gummed on the 

 edges. All the most important properties and defects of lenses may be thus 

 illustrated and studied. By means of a little extra care, two such lenses, ar- 

 ranged as we have described, in tubes blackened on the inside, and mounted 

 on a little wooden stand, the focus being adjusted by sliding the tube hold- 

 ing the lenses within another tube, also of paper, will give not only a very 

 fair view of such objects as the wing of a fly, the scales on a butterfly's wing, 

 and even the barbs on the sting of a bee, but it will show the globules of 

 blood quite distinctly, and we have even given a very interesting exhibi- 

 tion of the circulation of "the blood in the foot of a frog by means of a 

 temporary arrangement of this kind, which we put together for the pur- 

 pose of explaining to a little girl the construction of the microscope. Wo 

 would not recommend any one to use such a microscope for purposes of 

 work or study, because the fallacies to which it may give rise are too 

 numerous and too serious. But any boy, or even girl, who will undertake 

 the construction of such an instrument, cannot fail to obtain thereby an 

 amount of information which the perusal of volumes would not give. As 

 hints towards aiding our young friends, we may remark that our tnbes 

 were made of the best stiff paper, rolled up tight and pasted only along 

 the outer edge. The lenses were secured in their places by being attached 

 to the bottoms of pill-boxes, holes being punched through to admit the 

 light. Pill-boxes with holes were also used for diaphragms to reduce the 

 effects of aberration. A piece of mirror reflected the light, and the sides 

 etc., of an old cigar box furnished material for the stand. Fifty cents 

 covered all expenses. 



