22 PROPERTIES OF LENSES. 



their lives immersed in matter." A shout of astonish- 

 ment is raised at an old idea, no doubt in a new 

 dress, and the great discovery itself is missed in the 

 delight of at last conceiving what has been more 

 or less dimly perceived for centuries. 



The great discovery is, that the microscope does 

 not act dioptrically under certain circumstances, and 

 that, we have obtained a new aid to research in these 

 diffraction phenomena, which hold almost as high a 

 position in microscopical investigation, as spectro- 

 scopy in astronomy. 



We hope that these explanations will enable the 

 reader to choose an objective suitable for the parti- 

 cular branch of study he may be engaged in, it must 

 however be remembered that for low powers and the 

 majority of pathological investigations very large 

 angular aperture is not considered advantageous, as 

 with every increase of it the penetration and working 

 distance decreases, also that the extra amount ex- 

 pended in the purchase of first-class objectives, will 

 produce a considerable saving in subsequent expenses, 

 on account of the increase of successful results due to 

 their employment. The amateur is therefore strongly 

 recommended to patronise only the best makers, and 

 pay the highest price for his objectives. 



Very little time will be devoted to the consideration 

 of the principles involved in the formation of the 

 image in the compound microscope, as we are not 

 directly concerned with it in photo-micrography. 



