82 PEACTICAL PHOTO-MICEOGKAPHY 



has of forming an image. The tests already mentioned, while 

 in no way entitled to be regarded as exhaustive, may assist 

 an ordinary observer in gaming some information as to the 

 extent to which his objectives have this property. The 

 illustrations (Fig. 8, a, b } c, d) will give some idea of the appear- 

 ances that may be expected when using certain objectives in 

 combination with a particular ocular. They are all photographs 

 of the Abbe test-plate. Fig. 8 a is taken with a one-sixth-inch 

 achromatic objective of undoubtedly good quality, in com- 

 bination with a No. 2 projection ocular. For comparison, 

 Fig. 8 b has been taken under exactly similar conditions, 

 but a 4-mm. apochromatic has been used instead of the 

 one-sixth -inch achromatic. The difference between them both 

 in definition and general brilliancy of the image is at once 

 apparent, and will give a general idea of the appearance that 

 may be expected when observations are made for the purpose 

 of determining the quality of objectives. 



Figs. 8 c and 8 d are taken under similar conditions, except 

 that projection-ocular 4, in conjunction with a somewhat 

 greater camera extension, has been used. Fig. 8 c is taken 

 with the achromatic, and Fig. 8 d with the apochromatic 

 objective. Here again the difference can be seen immediately, 

 and owing to the higher power ocular employed is decidedly 

 accentuated. 



Another point worthy of note in these figures is the differ- 

 ence in centration that occurs with different objectives. The 

 two objectives were screwed into the same sliding objective- 

 changer and were then placed on the microscope : when 

 Figs. 8 a and b and also c and d are compared, it will be 

 seen that a totally different field of view has been included by 

 each objective. Thus the difference in centration is in this 

 particular case at least equal to the area of the 'whole field. 

 This is by no means an exaggerated instance, and is a state 

 of affairs that may be expected when any considerable number 

 of objectives are being used in succession. It demonstrates 

 clearly the necessity of using a method of changing the 

 objectives which will admit of their correct centration to one 

 another and to the optical axis of the microscope. 



A good selection of objectives for photo-micrographic work 

 would be the following : 



