WILSKA LOW-VOLTAGE MICROSCOPE 



charges from accumulating on the photo- 

 graphic emulsion during the exposure, there 

 is a tiny generator of positive ions in the 

 image chamber. 



The change of specimen is both simple and 

 rapid. The specimen cap is inserted into the 

 hole on a rod-like support. Before entering 

 the high vacuum the specimen passes 

 through a fore-vacuum zone. The little 

 amount of air in the specimen cap escapes 

 into the fore \'acuum, and the high vacvmm 

 remains practically intact. For this reason 

 there is no waiting when changing the speci- 

 mens. 



The microscope can be operated in any 

 position between vertical and horizontal. 

 Tilting does not disturb the image on the 

 screen. Visual observation of the image can 

 be made more accurate by using a magnify- 

 ing lens or an auxiliary light microscope, the 

 latter of which can be put in place in 2-3 

 seconds. The use of a special fine-grain 

 viewing screen in the electron microscope 

 makes optical magnification of 50-100 times 

 practical. At low voltages there is very Httle 

 of the ''splashing" of scattered and second- 

 ary electrons that would seriously limit the 

 possibilities of similar high optical magnifi- 

 cation at 50-100 kV. For the same reason a 

 high photographic enlargement of low volt- 

 age electronic images is made possible. The 

 35 mm film used is non-perforated, and 

 about 160 pictures can be made without re- 

 loading the camera. 



Although ultimate refinements of the 

 instrument are still being made, numerous 

 kinds of specimens have been examined 

 with it, using 18 kV as an accelerating po- 

 tential. The main difference between images 

 obtained with this voltage and the usual 50- 

 100 kV is the strength of contrasts. Many 

 structures that are too thin or too weak 

 when viewed with the usual voltages are 



much better visualized with this lower 

 voltage. On the other hand, many specimens 

 that are ideal for 50 to 100 kV are quite too 

 thick for our instrument, a circumstance 

 applying to supporting membranes as well. 

 Fortunately, it is easy to make membranes 

 that cast very Httle shadow even at 18 kV. 

 It is somewhat more difficult to make sec- 

 tions thin enough with the present ultra- 

 microtomes. 



At the present, the resolving power of the 

 microscope is about 25 A which is quite good 

 considering the greater wavelength asso- 

 ciated with the lower velocity of the elec- 

 trons used. 



To quote Dr. Frank N. Low: "Many 

 biologically significant macromolecules such 

 as those of hemoglobin and pepsin possess 

 a size well within the resolving power of 

 current electron microscopes, but have never 

 been visualized because of contrast problems. 

 A low voltage electron microscope is able to 

 produce images of such macromolecules 

 without the previous use of heavy-metal 

 shadowing or any structural alteration of the 

 specimen such as staining. This should open 

 up a vast field of research in the visualization 

 of organic substances at the macromolecular 

 level of structural organization." 



As encouraging as results have been so 

 far, the real era of low-voltage electron 

 microscopy will first come when its supe- 

 riority in contrasts can be combined with a 

 resolving power of only a few Angstrom 

 units. This will enable observation of the 

 elementary shapes of most of the biological 

 structures, e.g., polypeptide chains. There is 

 no hope of doing this without first correcting 

 at least a part of the tremendous spherical 

 aberration inherent in all existing electron 

 lenses. 



Alvar p. Wilsk.\ 



315 



