366 The Naturalist of Cumbrae. 



TYPE-SLIDES. 



" When working at the post-tertiary clays, I thought that it 

 would be an advantage to have a slide that would hold all the 

 ostracoda or foraminifera of a deposit, so that those of one 

 deposit could be compared with those of another with the least 

 possible trouble. For this purpose I made one of the ordinary 

 slides into an oblong cell. At first I made it on the hard-wood 

 slide, but I found the cardboard much more easily managed, 

 and it looked better, and conformed with the rest of my slides. 

 It held conveniently for examination, twenty to twenty-five 

 species on each half of the slide, and this could be increased to 

 double the extent by placing two species on one line, separating 

 them by red spots. These slides were found very convenient 

 when I was working conjointly with Dr. G. S. Brady." 



Sf^S3! 5US 



Diagram of type-slide, actual size. 



RULED SLATE. 



When a mass of fine material is placed under the microscope, 

 from which the observer desires to select here and there certain 

 minute objects, and when after careful examination of part of 

 the field he has found and removed one of these objects, it often 

 happens that his eye cannot again find the spot at which its 

 search was interrupted, and much time is wasted by going over 

 the old ground once more. To remedy this, Robertson provides 

 himself with a small oblong slate, suitable for use on the stage 

 of a microscope, and upon this slate he rules parallel lines and 

 between the lines inscribes arrows which point in opposite 

 directions on the alternate spaces. Hence in searching for 

 foraminifera among sand spread over the slate, he has but to 



