```r
physics %>% 
  filter(str_detect(text, "AK")) %>% 
  select(text)
```

```
## # A tibble: 34 x 1
##    text                                                                  
##    <chr>                                                                 
##  1 Now let us assume that the ray has come from A to C along AK, KC; the 
##  2 be equal to the time along KMN. But the time along AK is longer than  
##  3 that along AL: hence the time along AKN is longer than that along ABC.
##  4 And KC being longer than KN, the time along AKC will exceed, by as    
##  5 line which is comprised between the perpendiculars AK, BL. Then it    
##  6 ordinary refraction. Now it appears that AK and BL dip down toward the
##  7 side where the air is less easy to penetrate: for AK being longer than
##  8 than do AK, BL. And this suffices to show that the ray will continue  
##  9 surface AB at the points AK_k_B. Then instead of the hemispherical    
## 10 along AL, LB, and along AK, KB, are always represented by the line AH,
## # ... with 24 more rows
```