"SOLID- IMAGE" MICROSCOPE 



Fig. 1. Gregory "solid image" microscope: left, microcsope in which slide is mounted on steel tuning 

 fork (not shown); right, screen mounted on matched tuning fork upon which synchronous vibrating 

 image is projected. The 2 processes thus provide scanning of specimen in depth giving a "solid image." 



depth of field, and whicli presented the image 

 as a solid in a luminous block in which the 

 structures could be observed in depth would 

 have important advantages over existing 

 optical microscopes. We have designed and 

 constructed a primitive prototj^De of an 

 instrument giving such a "solid image". It is 

 possible to observe this from any position 

 and to see the structures with appropriate 

 parallax. 



The instrument involves two processes. 

 (1) The focal plane of the objective is effec- 

 tively made to "scan" up and down through 

 the depth of the section. In the prototype 

 instrument, this occurs at a rate of 50 double 

 scans/sec. The slide is mounted on a steel 

 fork tuned to 50 c./s. which is driven by a 

 polarized solenoid energized b}^ a 50 c./s. 

 sine wave. Thus the slide is carried up and 

 down through the focal plane of the objective 

 50 times/sec. (2) The image is projected on 

 to a screen mounted on a second tuned fork 

 which vibrates at the same rate and in the 

 same phase as the slide carrying the speci- 

 men. 



Scanning serves to extract the information 

 in depth from the specimen ; the second proc- 

 ess reconstitutes it in depth, giving a "solid 

 image" in the space swept by the screen. Its 

 frequency of vibration is greater than the 

 fusion frequency for the eye, so that little 

 or no flicker is observed. 



Figure 1 is a photograph of the apparatus, 

 in which the tuning fork does not appear. 

 In improved designs an aperiodic drive is 

 superior to the power-economizing tuned 

 fork, for this will not drift in phase with 

 temperature changes as does the tuned fork. 

 The vibrating screen is replaced b}^ a helix 

 driven by a synchronous motor, the image 

 to be projected on a sector near the edge of 

 the helix. A solid glass helix or "circular 

 wedge" is being experimentally tried for 

 changing the focal length of the objective to 

 give scanning without physical movement of 

 the objective or the specimen. With a sinu- 

 soidal scan, as used now, it is essential (a) 

 to modulate the intensity of the light by 

 using a high-pressure mercury lamp off 

 mains, and to phase this with velocit}^ ; and 

 (b) to chop the hght at each half-scan, to 

 cut out the image produced either by the 

 upward or downward scan to prevent the 

 possibility of mis-registration of the 2 sets of 

 images given by a phase error or by asym- 

 metrical wave form of the scan. The sub- 

 stage condenser is extremely important; it 

 must be of high N.A. (numerical aperture) 

 and correctly adjusted. 



The instrument was exhibited for the first 

 time in London on May 23, 1960. 



REFERENCE 

 Naiure, 182, 1434 (1958). 



Richard L. Gregory 



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