WITH THE MICROSCOPE. 7' 



part of the field to another without altering its distance from the 

 object-glass. If the field is flat, the object will appear equally well- 

 defined in all parts, but if the glass is defective in this particular, an 

 object accurately focussed in the centre will be found to be blurred 

 and out of focus when it is moved to the circumference. Or a stage 

 micrometer, 60, ruled to hundredths and thousandths of an inch 

 may be brought into focus. If the lines are sharp and clear, and per- 

 fectly parallel with one another in every part of the field, the glass is 

 a good one ; but if some appear curved and thicker at the circum- 

 ference of the field than at its centre, the glass is defective. 



It is not to be supposed that, even if the most minute directions 

 were given, the student just commencing work would be able to test 

 the object-glasses he was about to purchase, in all necessary par- 

 ticulars. Generally he may trust the maker, but if he desires to 

 ascertain if his object-glass is good, perhaps the simplest plan is to 

 compare the images produced by the same object first placed under 

 his own power and then under a glass magnifying in the same 

 degree, but of known excellence. 



9. Angle of Aperture. For ordinary work it will be found in- 

 convenient if the object-glass, when in focus, comes too close to the 

 object. This is a defect in glasses having a high angle of aperture. 

 The angle of aperture is the angle made by two lines from opposite 

 sides of the aperture of the object-glass with the point of focus of the 

 lens. The angle B A B in fig. 8 is the angle of aperture. Glasses 

 with a high angle of aperture admit much light, and define many 

 structures of an exceedingly delicate nature, which look confused 

 when examined by ordinary powers, but for general work I recom- 

 mend glasses of medium angular aperture. 



Mr. Ross has lately made glasses having an angle of 170 degrees, 

 which are valuable for investigations upon many very delicate and 

 thin structures, such as the diatomaceas ; but such powers are not 

 well adapted for ordinary work. The importance of arranging the 

 object very carefully and the necessity of paying great attention to 

 the adjustment and illumination, render these glasses inconvenient 

 for general observation. The penetrating power of glasses with a low 

 angle is much greater than in those of a high angle of aperture, so 

 that exact adjustment is much more important in the latter than in 

 the former. 



The refraction produced by the passage of the light through the 

 thin glass covering the object varies according to its thickness, and 

 it has been found necessary to render the higher powers capable of 

 being adapted to this varying refraction. It is especially necessary 

 in glasses of high angle of aperture, and is usually effected by 



