I have previously explained how Ed Ricketts used a 20X Bausch & Lomb hand magnifier. However, this claim was based purely on a quote from John Steinbeck. Prompted by a question from Eric Enno Tamm, I have just searched through Between Pacific Tides by Ricketts using Google books and there are 9 specific mentions of using a hand lens to reveal the beauty of what can be seen in the tidepools. Some of these descriptions are poetic - here are some examples.
Page 59. §29 - Tegula brunnea & Crepidula adunca
With a hand lens the embryos of Crepidula can sometimes be seen whirling around in their envelopes in the egg packets.
Page 85. §62 - Transparent shrimp Heptacarpus pictus.
There is such a fairylike beauty to this ephemeral creature that the inexperienced observer will be certain that he is seeing a rare form...
Once captured, the living specimen should be confined in a glass vial not much larger than itself and examined with a hand lens. The beating heart and all the other internal organs can be seen very plainly through the transparent body.
As would be expected in a haunt so prolific as the lower tidepool zone, the shelter of the hydroid forests attracts a great many smaller animals, both sessile and active. In this work, it is difficult to say at just what point animals become too inconspicuous to be considered, for in the tidelands it is almost literally true that
Great fleas have little fleas
Upon their backs to bite 'em
And little fleas have lesser fleas
And so on ad infinitum.
... Though all of these animals are at least visible to the naked eye, and are abundant, characteristic and certainly not lacking in interest, most of them are too small to be seen in detail without a hand lens or microscope and hence cannot be included in this handbook.
Page 143. §110
On the "leaves" and "stems" of outer-tide-pool kelps, one almost always finds an encrusting white tracery delicate enough to be attributable to our childhood friend Jack Frost. But a hand lens reveals a beauty of design more intricate than any ever etched on frosty window-panes. These encrustations are usually formed by colonies of the bryozoan or ectoproct, Membranipora membranacea (Fig 114), so named in the middle of the eighteenth century by Linnaeus, regarded as the founder of modern classification.
The minute, calcareous cells, visible to the keen naked eye but seen to better advantage with a lens, radiate in irregular rows from the centre of the colony.
...the process of larval settlement has been described for several species. The process can be watched by any careful observer with a good lens.