160 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



without screw- collars, and they require to be used 

 with thickened cedar -wood oil, having a refractive 

 index of ±'5128. In spite of their large apertures, 

 they will work through covers of about T J^ in. 

 thickness. 



The lenses kindly submitted to me for inspection 

 are the following : 



mm. focus. mm. aper. 



Apochromatic objective, 16 about £ in. and 0*30 



,, ,, 4 ,, fin. ,, 0-95 



,, water immersion, ,, 2*5 „ T Vin. ,, 1*25 



,, homog. ,, ,, 2 ,, T Vin. ,, T40 



They are undoubtedly the finest objectives which 

 I have ever seen, leaving far behind in their 

 performances many lenses which I have hither- 

 to considered as almost not to be surpassed. The 

 pictures produced by these new lenses are re- 

 markably achromatic and bright, and owing to the 

 exquisite definition of these objectives structural 

 details on surface markings come out with wonder- 

 ful sharpness and distinctness. I am especially 

 pleased with the lens of 4 mm. focus and the 

 homogeneous immersion lens of 2 mm., which latter, 

 owing to its enormous aperture and brilliant defini- 

 tion, resolves test objects, considered difficult, with 

 the greatest ease. The facility with which high 

 magnifications may be obtained without loss of 

 definition, and without discomfort to the eyes by 

 using high eyepieces, is most agreeable. I have 

 no hesitation in saying that these apochromatic 

 objectives are destined to supersede the present 

 achromatic objectives, and that as Dr. Carl Zeiss 

 offers his new optical glasses to the trade, and, 

 with his usual liberality, places no restrictions, by 

 patents or otherwise, on the making of these lenses, 

 other opticians will soon enter into wholesome 

 competition with him, with the result that apo- 

 chromatic objectives will shortly be supplied by all 

 good makers. Meanwhile the high prices of the 

 apochromatics are standing in the way of their 

 general adoption, the lower powers costing almost 



