182 THE AMERICAN MONTHLY [August, 



Selecting a Microscope. 



By G. S. WOOLMAN, 



NEW YORK. 



There is a simple instrument which, with its three lenses combined, 

 has a power of thirty-three diameters, which sells for $3.50. With it 

 can be seen many of the larger animalcule in pond water, the scales 

 from a butterfly's wing, pollen-grains from plants, and thousands of 

 other objects not visible to the naked eye. From $3.50 the prices for 

 microscopes range up to $350 and $400. There are many different 

 styles and grades, a very common mistake made by persons attempting 

 to select a microscope is to judge of the excellence of an instrument by 

 the amount of its magnifying power. No object should be viewed with 

 a power greater than that required to show its structure, and if that 

 can be done with thirty diameters it is, to say the least, unnecessary to 

 use one hundred. This is especially the case with low-priced instru- 

 ments, where the apertures of the objectives are small and the connec- 

 tions not so exact as in the higher grades, rendering them more liable 

 to give false impressions of objects. Moreover, it is absolutely impos- 

 sible to view opaque objects satisfactorily by the reflected light of cheap 

 compound microscopes. For those who wish to dissect flowers and 

 insects for examination a simple instrument is better. 



In selecting, a microscope the essential points to be observed are that 

 the lenses show objects clear and well-defined, that the stand be of good 

 material and workmanship, and that there be no lateral movement in 

 the adjustments of the focus. Further, that the focus be instantly 

 changeable when desired, and that it have a joint for inclination. As 

 to the different kinds of microscopes. The simplest, of course, is the 

 single glass, such as is used by watchmakers and engravers, and the 

 common pocket glass with from one to three lenses. The simplest 

 microscope "with a stand is the one mentioned above for $3.50; with 

 its three lenses combined it has a magnifying power of 33 diameters. 

 It packs in a box that acts as a base for the upright brass stem. With 

 it comes an animalcule cage, a pair of brass forceps, a watch-glass, two 

 plain glass slips, and a prepared object. The school microscope is 

 similar, but works easier, and is better adapted for school purposes. 

 Of the compound microscopes, one sells for $2.50, which is the simplest. 

 It is of polished brass with one piece and one object-glass, magnifying, 

 when combined, about 40 diameters (or 1,600 times), the power being 

 calculated by squaring the diameter. 



A powerful instrument for household use, with its two object-glasses, 

 magnifies from 30 to 100 diameters, and ranges in price according to 

 size and quality from $5 to $12. For ordinary use, an amateur micros- 

 copist can buy an instrument for from $23 to $30 which will answer 

 his every purpose. Such a microscope will have a stage with adjust- 

 able spring clips, a revolving diaphragm with four apertures beneath 

 the stage, and a concave reflecting mirror for use under or above the 

 stage. It can magnify 165 diameters, and, with the addition of a one- 

 fifth object-glass, this can be increased to 350 diameters. For students 

 in histology and vegetable anatomy there are instruments that range in 

 price from $50 to $400, and when one of the cheapest is furnished with 

 condenser, polariscope, camera lucida, spot lens, zoophyte trough, live 

 box, and forceps, it is complete for almost any investigation. 



