Dead-end design
Once Leeuwenhoek had the instrument, he had to learn how to use it. Using it was so difficult that his design was never used by anyone else to make important discoveries. In the history of the microscope, it was a dead end.
- It was too hard to make.
- It was too hard to use.
But it let Leeuwenhoek see things that it took until the mid-1800's for the double-lens microscope to reveal to Pasteur and Lister. Not until these scientists developed the germ theory of disease did they begin to understand the significance of the bacteria and protozoa discovered two centuries previously by Antony van Leeuwenhoek.
In the same paragraph of Micrographia's Preface where Robert Hooke told how to make a glass bead lens and mount it behind a brass plate, he wrote about why he did not use it, in spite of its superiority to the double lens microscopes he used for the observations in his book:
These, though exceeding easily made, are yet very troublesome to be us'd, because of their smallness, and the nearness of the Object."
Almost two decades later, in Microscopium, he returned to the topic of single lens microscopes. In the meantime, he had learned that they were not "exceedingly easily made". But they were still hard to use:
... to make any of a sphere less than 1/10 of an inch in diameter is exceeding difficult, by reason that the glass becomes too small to be tractable; and 'tis very difficult to find a cement that will hold it fast while it be completed; and when 'tis polished, 'tis exceeding difficult to handle and put into its cell.
Besides, I have found the use of them offensive to my eye, and to have much strained and weakened the sight, which was the reason why I omitted to make use of them, though in truth they do make the object appear much more clear and distinct, and magnify as much as the double microscopes.
nay, to those whose eyes can well endure it, 'tis possible with the single Microscope to make discoveries much better than with a double one, because the colours which do much disturb the clear vision in double Microscopes is clearly avoided and prevented in the single.
On June 9, 1699, midway through his career with them, Leeuwenhoek wrote to the Royal Society:
As to what concerns my Magnifying glasses, I will not brag of them, I make them as good as possible I can in my power, and I must say that several Years since, I have not only Ground them still better and better, which is a matter of consequence, but I have also mounted them better from time to time, which is also very Material.
How did Leeuwenhoek use the microscopes to solve his problems?
Every design decision involves a trade-off. Leeuwenhoek's design solved most, but not all of the problems posed at the beginning of this section on Leeuwenhoek's microscopes. In doing so, it sometimes created other problems.
Hold the lens
The plates held the lens tightly but not too tightly. He could vary the size of the lens hole to work better with a particular lens and specimen. However, he could not swap out lenses. The aalkijker that he pictured in one of his letters had only one fixed lens, but the design made by Musschenbroek that is in the Boerhaave has interchangeable lens, so Leeuwenhoek must have used a similar design to be able to swap out the 172 lenses that were auctioned after his death.
Keep the specimen and the eye within the very short focal length
The L-bracket kept the specimen pin close. A dry specimen would be larger than the pin and thus even closer to the lens. A section of capillary tube could be glued in front of, beside, or behind the pin. On the other side, there was nothing to keep Leeuwenhoek's eye from getting within a millimeter or two of the lens.
Focus the specimen
After the specimen was fixed to the pin, the positioning screw could move it in two directions. Assuming the pin was pointing upward, turning the handle would move the mount and thus the specimen up and down. Because the positioning screw could pivot off the braking screw, it could also move the specimen back and forth. The focus screw would move it in and out, closer to the lens or farther away.
Although the screw threads of this soft metal would soon wear and make focusing difficult, there were a lot of threads, making fine adjustments possible.
Retain the focus
When Leeuwenhoek had the specimen in focus, a quick turn of the braking screw would freeze it. Because it had to hold three pieces of metal close together, the L-bracket and the riveted body plates, the braking screw had a nut.
Light the specimen
This design did not solve his lighting problem very well. It helped that the microscope could be easily angled toward or away from a light, such as a candle or open window. However, an opaque object had to be too close to the lens. Perhaps that explains the ratio of silver lens; they could be polished to reflect at least a little light back on the observable side of an opaque specimen.
Keep his hands free
Holding one of these microscopes to the eye for an hour becomes an increasingly arduous task.
