Marvels of Pond- Life. 5 



power is exceedingly well adapted for the examination 

 of living insects, or of the exquisite preparations of 

 entire insects, which can now be had of all opticians. 

 Microscopes which have a draw tube can be furnished 

 with an erector, an instrnment so called because it 

 erects the images, which the microscope has turned 

 upside down, through the crossing of the rays. This is 

 very convenient for making dissections under the in- 

 strument ; and it also gives us the means of reducing 

 the magnifying power of an object-glass, and thus 

 obtaining a larger field. The erector is affixed to the 

 end of the draw tube, and by pulling it out, or thrusting 

 it in, the rays from the object-glass are intercepted at 

 different distances, and various degrees of power ob- 

 tained. 



A binocular microscope is most useful with low 

 powers from two thirds upwards. A new form, devised 

 by Mr. Stephenson, acts as an erector, and is very 

 valuable for dissections. It works with high powers. 



Beginners will be glad to know how to obtain the 

 magnifying power which different objects require, and 

 it may be stated that, with a full-sized microscope, a 

 two-inch object-glass magnifies about twenty-five dia- 

 meters with the lowest eye-piece; a one-inch object- 

 glass, or two thirds, from fifty to sixty diameters ; a 

 half-inch about one hundred ; a quarter-inch about two 

 hundred. The use of deeper eye-pieces adds very con- 

 siderably to the power, but in proportions which diflcr 

 with different makers. One instrument used by the 

 writer has three eye-pieces, giving with a two thirds 

 object-glass powers of sixty one hundred and five, and 



