510 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



ment, greatly injures the proper performance of the latter. The third 

 objection is that spectacle-glasses add two light refracting and re- 

 flecting surfaces to those already existing. It is almost impossible 

 for the observer wearing spectacles to even roughly place the optic 

 axis of the spectacle lens, if worn in the ordinary manner, in line with 

 that of the instrument. 



On the other hand, Mr. J. Martin finds * that " in every case 

 where test objects could be seen both with and without the spectacles, 

 the definition was better when they were used." 



Immersion Objectives. f — Mr. E. Gundlach has a wonderful paper 

 under this title, which carries one back to the dark ages of microscopy. 

 The following is quoted verbatim : — 



" The refractive power of water being much lower than that of 

 glass or homogeneous oil, it will, if put in place of those substances, 

 exert a correspondingly smaller influence in correcting the aberra- 

 tions. But, on the other hand, while the use of the homogeneous 

 medium permits the preservation of the full working distance without 

 any loss in correction, this loss, if water be employed, can, in a great 

 degree, be regained if so much of the working distance as can be 

 spared is sacrificed and the space filled with glass. This can best 

 be done by adding to the thickness of the front lens so much that 

 only just enough of the working distance is left as is practicable, 

 and then fill the comparatively small immersion-space with water. 

 Indeed, by a skilful balancing of the interfering conditions, the dif- 

 ference between the adaptation of water and homogeneous oil can be 

 reduced to a minimum, and yet the working distance be as long as is 

 practically required. 



" The high optical superiority of the modern homogeneous im- 

 mersion objectives over the old water-immersion may seem to dis- 

 prove this theory. But I do not hesitate to claim right here that the 

 wonderful performance of these objectives is due in a comparatively 

 small degree only to the homogeneous immersion ; it is due in a far 

 greater degree, to the increase of the number of lenses and, conse- 

 quently, the number of refracting surfaces. We remember that at the 

 same time as the homogeneous immersion the four-system principle 

 was introduced. Probably a more important advantage of the homo- 

 geneous over the water-immersion, than that of the higher corrective 

 power, may be found in the fact that adjustment for cover-thickness 

 is unnecessary. But even this merit is doubted by many first-class 

 authorities on the manipulation of the Microscope, and the demand 

 for adjustable homogeneous objectives is on the increase. 



" Under such circumstances, weighing its merits and its faults, it 

 must be admitted that the practical advantages of the homogeneous 

 immersion principle are at least doubtful. This cannot be said of 

 the four-system principle. It is unnecessary to enter into a thorough 

 theoretical investigation of this matter. It may suffice to call to 

 mind the fact that the aberrations of higher order are inversely pro- 



* The Microscope, vi. (1886) pp. 79-80. 



t Proc. Amer. Soc. Micr., 8th Ann. Meeting, 1885, pp. 51-3. 



