452 Transactions of the Society. 



monochromatic l! aid screen being required to shut the blue part 

 of the purple oui ' * Vet this lens gives particularly sharp images, 

 and is a very strong diatom resolver. Now, however, Herr Winkel 

 has revived the American red corrections with Jena glasses. The 

 resull is excellent, for brighter, sharper, or, for their apertures, 

 mger resolving object-glasses will not be found. This red 

 correction is peculiarly suitable, because a peacock-green glass 

 .11 turns red info black, and so makes a strongly contrasted 

 image. When the Podura was first examined with the } of 0*85 

 N.A., for the moment it was difficult to exclude the idea that one 

 of the American red objectives was not on the nose-piece. 



Leaving the achromats, we come to the fluorites.t These form 

 a beautifully corrected series of object-glasses — indeed, they might 

 appropriately be termed -| apochromats, so small is the outstand- 

 ing secondary colour ; what there is, is of the same red tint. These 

 glasses give exceptionally brilliant and sharp images. The 1 ' 8 mm. 

 fluorite oil-immersion gave an image that was only surpassed by a 

 very superior apochromat ; the 8 ■ 5 mm. is also a beautiful objective. 

 All these glasses are, as they ought to be, extremely sensitive to 

 tube-length, a very little amount either one way or another is 

 sufficient to make or mar the image ; the tube-length, be it noted, 

 is 170 mm., an increase of 10 mm. in the right direction, for which 

 we ought to be thankful. All the object-glasses were tested both 

 by daylight as well as by lamp-light, in order that their chromatic 

 conditions should be thoroughly explored. 



Now, as to the eye-pieces, those of the compensating series are 

 of the usual over-corrected type, and are carefully adjusted for the 

 corrections of the object-glasses, a condition which, it is to be feared, 

 is in >t always satisfied. As regards the next series, a new word, 

 " complanat," has been coined to denote this new eye-piece. It is 

 of the Huyghenian type, but differs from it in being strictly achro- 

 matic, and in having a perfectly flat field. The examination of a 

 microphotograph of printed matter under a low power will at once 

 demonstrate the superiority of this form of eye-piece. Complanat 

 N". 5. compensating X 18 for the long tube, and compensating 

 X 12 for the short tube, have about the same power. 



In conclusion, Winkel's high finish is still preserved ; never has 

 such beautifully finished brass and glass work been put into a 

 Microscope objective ; it is not only a pleasure to look through 

 it, but, to any one taking an interest in this kind of work, it is also 

 a pleasure to look at it. It is finish, and not polish, which is 

 meant. It has been said that Wansehaff (alau ! now no more) and 

 W inkel hold a record against the world for their magnificent brass 

 work. 



* A delicate test for colour is the raphse of a Cherrvfield Rhomboides, when 

 mounted in balsam, quinidine, or styrax. 



t Y\ inkel claims priority for the introduction of this class of objective. 



