Observations on Amjihijjleura pellucida. By J. J. Woodivard. 673 



No. 5. Glycerine-immersion ^, aperture 105°, by Spencer. 2830 diameters. 



No. (j. Glycerine-immersion i, aperture 106°, by Spencer. 1900 diameters. 



No. 7. Enlarged from the last negative fo 27(J0 diameters. 



No. 8. Water-immersion yL., aperture 91°, by ToUes. 27G0 diameters. 



No. 9. Water-immersion i, aperture 105°, by Powell and Lealand. 2700 

 diameters. 



No. 10. Water-immersion ^'g-, aperture 103°, by Powell and Lealand. 2700 

 diameters. 



No. 11. Water-immersion Jg-, aperture 91°, by Powell aud Lealand. 2900 

 diameters. 



(6) Photographs of Amphipleuea pellucida, illuminated by monochromatic sun- 

 light, with an immersion illuminator and the utmost obliquity each objective would admit 

 without distortion. 



No. 12. Zeiss' oil-immersion —^ (same as No. 1). 2830 diameters. 

 No. 13. Tolles' oil-immersion J^j (same as No. 4). 2760 diameters. 



From my examination of these objectives, I am constrained to 

 give the Zeiss yV the preference, both by lamplight and sunlight, 

 over all the others named, and, indeed, I may add, over all the 

 objectives I have ever yet examined. 



Next comes a group embracing the oil-immersion ^ by Tolles, 

 the glycerine-immersion ^ and ^^ by Spencer, and the oil-immersion ^ 

 by Zeiss. All these objectives perform very well indeed. When 

 I wrote to Zeiss last January, I expressed the opinion that the per- 

 formance of his ^ fully equalled that "of the best of the large 

 collection of immersion objectives belonging to the ^luseum." But 

 subsequent trial convinced me that my first photographic work with 

 the Spencer's ^V ^^^ ^o^ A.onQ it justice, and I afterwards received 

 the ^ from the same maker and the oil-immersion ^^ by Tolles. 

 After protracted trials, I now regard these three objectives as supe- 

 rior in defining power to the Zeiss ^. How they compare with it, 

 and with each other, may be fairly judged from the photographs. 

 Of the water-immersion objectives, the Tolles ^V stands first in my 

 estimation ; the Powell and Lealand objectives next. 



Among other points suggested by a study of these photographs 

 is the fact that the superiority of the glycerine and oil-immersion 

 objectives is not a mere consequence of their great aperture. The 

 aperture of the ^ of Spencer exceeds but little, and the ^l not 

 at all, that of the ^ of Powell and Lealand, yet their perform- 

 ance is much better. The aperture of the Zeiss J^ is actually 

 less than that of the Tolles y^, which, however, it excels in per- 

 formance, and a similar comparison may be made between the 

 Tolles xV and the Powell and Lealand objectives. And yet I do 

 not doubt in the least that each additional degree of interior angle 

 above 82^ is a material advantage, provided always that the aber- 

 rations of the objective are accurately corrected ; but inferiority in 

 the formula employed or in the skill and care exercised in construc- 

 tion may more than neutralize the advantages that ought to be 

 derived from this source. Nor do I doubt in the least the 



