176 THE AMERICAN MONTHLY [Juu 



tlie correction collar or .'uijustraeut of tube-length for dif- 

 ferences in tlie tliickness of cover-glass. This has been 

 fully dealt with in an earlier paper {Science-Gossip, Vol. 

 VI., N.S., p. 184); but it roally requires a trained and 

 critical eye, as well as a critical object,to enable these ad- 

 justments to be satisfactorily made. We may, however, 

 remind our readers that the oft-met-with instructions as 

 to varying the magnification by the simple device of al- 

 tering the tube-length are not really practicable save for 

 very rough-and-ready work, and we recommendourreaders 

 to find out at the time of purchasing whether their ob- 

 jectives are corrected for the G^-inch or 10-inch tube, and 

 to remember that they peiform properly only on the tube 

 for which they are designed. Most students' objectives 

 (for an unsatisfactory reason connected, we believe, with 

 foreign competition) are corrected for the short tube ; but 

 makers have a misleading habit of giving in their cata- 

 logues complete lists of magnifications calculated as if 

 they were corrected for the English 10-inch tube-length 

 and vouchsafing no hint as to the real facts. 



There are a few more suggestions that may be useful to 

 the beginner. The first is to remember never to use a 

 stronger light than is necessary. Nothing is more fatig- 

 uing to the eyes, or more likely to work mischief, than 

 excessive glare ; but if reasonable precautions be taken, 

 we do not believe the use of the microscope is injurious 

 to the eyesight. Some people are much troubled with what 

 are called "floating flies" in the eye but this is to a cer- 

 tain extent a question of ease of position. The second rule 

 is to accustom oneself to keep both eyes open. The screw- 

 iuo--up of the eye not in use is a mo.st injurious and un- 

 necessary habit. At first, doubtless, some diflBculty will 

 be found, and the eye that is not looking down the tube 

 will be distracted by external intluences; but this dilfi- 

 cuUy is only temporary, and a little perseverance will 

 overcome it. Some writers recommend a shade, but we 



