248 



PROGRESS IN MICROSCOPY 



Setting on Ty the image B' of the vacant area B of the specimen, 

 the reading is Wq'. The transmission factor T^'^r of A' in relation 

 to B' is given by T^'^' = W^^IW^, whence the density d^^^' of the 

 detail A'. 



In the Thorell arrangement, the vibrating mirror m^ is set between 

 the objective O^ and the small aperture T^ (Fig. 10.7), the latter being 



Fig. 10.6. Double beam arrangement with photo-cell (P. M. B. Walker). 



alternately spread over by the imaged detail being studied and the 

 imaged vacant area of the specimen. It follows that the reference 

 beam, passing through W, is dispensed with (Fig. 10.6). 



T. Caspersson, who blazed the trail of ukra-violet microspeclro- 

 graphy, has devised an extensive equipment for surveying quanti- 

 tative cytochemistry applications (shown in simplified form in Fig. 10.8). 



The objective 0^ of the microscope images the specimen on 

 a diaphragm provided with the small aperture T, behind which is ihe 

 cell Ri. The comparison beam, extraneous to the microscope, enters 

 the second cell R.. The current 7^, generated by /?i, is automatically 



